Which MTB size should you choose: S, M, L or XL?

Updated on 5 November 2026. The recommended products are available at the time of this update.

To choose your MTB size from S, M, L and XL, start with your height and the model's size chart, then check that your inside leg measurement leaves clearance above the frame and that the reach lets you ride without feeling too stretched out or cramped. Size letters mean different things across brands and geometries: if you are between sizes, compare the measurements and try both before deciding.

How to choose

The right size should let you pedal comfortably and move around on the bike when the trail gets rough. Simply being able to reach the pedals by lowering the saddle is not enough.

• Measure your height: barefoot, with your back against a wall and looking straight ahead.
• Measure your inside leg: barefoot, hold a book between your legs, snug like a saddle, and measure the distance from the floor to its top edge.
• Check the exact size chart: look up the model and year, because a M can vary even between generations of the same bike.
• Compare the geometry: check reach, standover and stack, then assess your position on a test ride.

As an initial guide, many MTB place S at around 155-170 cm, M at 165-180 cm, L at 175-190 cm and XL at 185-200 cm. These ranges are indicative and deliberately overlap: the individual model's size chart takes priority and, at either end, you may need a XS or a XXL.

Reach, standover and geometry: what they mean

Reach is the horizontal distance between the bottom bracket and the top of the head tube. It helps you understand how much room you have when riding standing on the pedals, but does not describe the distance from the saddle to the handlebar on its own.

Standover is the height of the top tube above the ground at the point specified by the manufacturer. Compare it with your inside leg measurement and check in person that you have clearance when standing astride the bike: off-road, you need to be able to get off the saddle without hitting the frame.

• Stack: indicates the height of the top of the head tube relative to the bottom bracket and contributes to handlebar position.
• Seat tube angle: affects your pedalling position. Two bikes with the same reach can feel different when seated.
• Head angle and wheelbase: affect how the bike handles. A cross-country MTB and a trail bike can have different proportions despite carrying the same size letter.
• Seat tube and seatpost: must allow the correct saddle height for pedalling. With a dropper seatpost, you also need to check the minimum height and insertion depth.

Two people of the same height can have different leg, torso and arm lengths. This is why your inside leg measurement helps you assess clearance and saddle adjustment, while your position at the handlebar also requires a comparison of the geometry.

If you are between sizes

The smaller size tends to give you a more compact bike; the larger size tends to offer more room and a longer wheelbase. Neither is automatically the better choice.

• First adjust the saddle for pedalling, respecting the seatpost's insertion limits.
• When seated, check that you can reach the handlebar and brakes with relaxed shoulders and slightly bent elbows.
• Standing on the pedals, check that you can move your body without feeling restricted or pulled forwards.
• Try setting off, braking and making slow turns in a suitable space.

The stem and adjustments can fine-tune your position, but they cannot always compensate for a frame that is too long or too tall. Moving the saddle a long way forwards just to get closer to the handlebar can also worsen your pedalling position.

27,5 or 29 inch wheels: these are not the frame size

Wheel diameter and frame size are two separate measurements. A 29 inch MTB can have a XS frame, while a bike with 27,5 wheels can be too big for the person trying it.

Some models change wheel diameter according to frame size. The Trek Marlin 5 Gen 3 featured here, for example, uses 27,5 wheels in sizes XS and S and 29 wheels in the others: you still need to choose the frame based on your measurements.

Numbers such as 40, 44 or 48 also require the brand's size chart: do not automatically convert them into S, M or L. To understand which type of suspension you need, read Mountain bikes: how to choose between hardtail and full suspension.

Used size XS MTB and full-suspension size M MTB: what to check

If you are looking for a used size XS MTB, ask for the brand, model, year and a photo of the size marked on the frame. Do not infer the size from small wheels or the seller's height.

If you are looking for a full-suspension size M MTB, remember that full suspension means the bike has rear suspension, not a standard fit. Compare the model's reach and standover, and have the suspension adjusted to your weight before the test ride.

• Frame identity: compare the stated size, markings and original geometry.
• Modified components: check the fork, stem, handlebar and seatpost. Replacements can change your position and the bike's handling.
• Available adjustments: check that the saddle can reach your required height without exceeding the limits and that any dropper seatpost can be lowered far enough.
• Condition: have the frame, wheels, brakes, drivetrain and suspension checked; on full-suspension bikes, also check for play and inspect the linkage bearings.
• Test ride: assess clearance above the frame, access to the controls and freedom of movement.

A well-maintained used bike is still the wrong choice if you cannot adjust it to fit your body.

Our recommendations, available now

These options are all hardtail MTB. Compare the variants on each bike's product page and the relevant size chart before choosing.

Megamo Natural 35 Hardtail Mountain Bike

It is suitable for those looking to get started on the trails and is available in sizes S, M, L and XL. It combines an aluminium frame, 29-inch wheels, a 100 mm Suntour fork and a 12-speed SRAM SX Eagle drivetrain.

Trek Marlin 5 Gen 3 2027 Hardtail Mountain Bike

It is an option for dirt tracks, trails and off-road rides, with an aluminium frame and a 9-speed Shimano CUES rear derailleur. Sizes XS and S have 27,5-inch wheels, while the others have 29-inch wheels; the fork offers 80 mm of travel in XS and 100 mm from S to XL.

Berria Bravo Sport SX Hardtail Mountain Bike

For those looking for a hardtail with XCO Racing geometry, it is available in sizes XS, M and L. It has a HMR2X carbon frame, 29-inch wheels, a 100 mm RockShox Judy TK fork and a 12-speed SRAM SX Eagle drivetrain.

Bottecchia 107 Hardtail Mountain Bike 27.5 Shimano TY500 21-speed

It is designed for leisure and rides on dirt tracks, with sizes listed as 44 and 48. It has 27,5-inch wheels, an aluminium frame, a 100 mm fork and mechanical disc brakes: consult the specific size chart to choose between the two sizes.

You can compare other options in the MTB Bikes collection. For shipping information, see the shipping policy.

Let's find your size together in Casapulla

Evolution Bikes has been a family-run shop in Casapulla, in the province of Caserta, since 1974 and is now in its third generation. With a workshop, bike fitting and e-bike servicing, we can help you try the bike and adjust your riding position: visit the shop or contact us with your height, inside leg measurement and the model you are interested in.

Frequently asked questions

I am 175 cm tall: is a MTB M or L better?

On many models, you might fit a M, but 175 cm may also fall within the overlap between two sizes. Check the model's size chart, compare reach and standover height, and try the riding position with the saddle adjusted.

Is a 29-inch MTB suitable for a short person?

It can be suitable if the frame has the right geometry and dimensions. The 29-inch measurement refers to the wheel diameter: pay particular attention to standover height, reach and minimum saddle height.

How do I check whether a used size XS MTB is right for me?

Identify the model and year, check the size on the frame and consult the original geometry. Then check the clearance above the top tube, saddle height and distance to the controls in person, as well as the mechanical condition.

Does a size M full-suspension MTB fit like a size M hardtail?

Not necessarily: the size letter does not guarantee the same dimensions across models. Compare reach, stack and standover height, and try the full-suspension bike with the suspension adjusted for your weight.