Buying a bike online is one of the best ways to ride more and spend less. The one catch? You can't throw a leg over it first. The good news is that getting your size right is mostly about two numbers: your height and the bike's reach. Nail those and you're 90% of the way there.

This guide covers the full size run from Small to XL, which typically fits riders from about 155 cm (5'1") to 195 cm (6'5"). We're talking in general terms here, because geometry changes from model to model. Always check the specific size chart on the bike you're eyeing, including the 2026 Siskiu T8, before you hit buy.

Why Fit Matters More Than You Think

A bike that's too small leaves you cramped, with sore knees and twitchy handling. A bike that's too big feels like steering a couch: hard to flick around, hard to stop, and hard to feel confident on. Research on cycling biomechanics has repeatedly linked saddle height and position to knee loading and overuse injuries, which is why a proper fit is more than a comfort thing. See, for example, the review on bike fit and overuse injury in PubMed-indexed cycling medicine literature.

The Quick Height-to-Size Cheat Sheet

These ranges are a starting point. Brands vary, so treat them as a rough map rather than gospel.

• Small (S): roughly 155-165 cm (5'1"-5'5")
• Medium (M): roughly 165-175 cm (5'5"-5'9")
• Large (L): roughly 175-185 cm (5'9"-6'1")
• Extra Large (XL): roughly 185-195 cm (6'1"-6'5")

If you land between two sizes, don't panic. That's where reach, inseam, and riding style come in.

Reach: The Number That Actually Tells You How It'll Feel

What is reach?

Reach is the horizontal distance from the center of the bottom bracket (where the cranks spin) to the center of the top of the head tube (where the steering sits). In plain English: it tells you how stretched out you'll be when you're standing and riding.

Why reach beats seat tube length

Old-school sizing was all about seat tube length. Modern mountain bikes use longer reaches, slacker angles, and dropper posts, so the seat tube matters less. Reach is now the headline number for cockpit room.

A simple way to use it

Compare the reach figures across sizes on the chart. The step between sizes is usually around 20-25 mm. If you like a roomy, stable feel, lean toward the longer reach. If you prefer a nimble, playful bike, go with the shorter one. Most riders are happy somewhere in the middle.

Stand-Over Height: The Safety Check

Stand over the bike's top tube with both feet flat on the ground. You want a few centimeters of clearance so a sudden stop doesn't become a painful lesson. On modern bikes with sloping top tubes, this is rarely an issue, but it's still worth checking the number against your inseam.

How to measure your inseam

1. Stand barefoot with your back against a wall, feet about 15-20 cm apart.
2. Place a hardback book snug between your legs, spine up, like a saddle.
3. Mark the wall at the top of the book.
4. Measure from that mark to the floor.

Do it twice and take the average. Your inseam also helps you pick a seat post length and a dropper post size that works for you.

Between Sizes? Here's How to Choose

Size down if...

• You ride tight, twisty trails and want a playful, easy-to-manage bike.
• You're a beginner who values confidence at slow speeds.
• You have shorter arms or a shorter torso for your height.

Size up if...

• You mostly ride fast, open trails and want more stability.
• You have long arms or a longer torso.
• You plan to run a shorter stem or want room to grow as your skills improve.

Easy Tweaks After the Bike Arrives

Good news: a few small changes can fine-tune the fit without swapping the whole bike.

• Stem length: a 10 mm change shifts your reach by about the same amount.
• Handlebar rise and width: changes how upright and relaxed you feel.
• Saddle position: sliding it forward or back adjusts your effective reach.
• Saddle height: a common starting point is a leg that's nearly, but not fully, straight at the bottom of the pedal stroke.

Your Online Fit Checklist

1. Measure your height and inseam.
2. Find the size chart for the exact model and year.
3. Compare the reach and stand-over numbers, not just the S/M/L/XL label.
4. Think about how and where you ride.
5. If you're still stuck, ask the support team before you order.

The Bottom Line

Size labels are a handy shortcut, but height and reach tell the real story. Use the 155-195 cm range as your rough guide, double-check the chart for your specific bike, and remember that stem and saddle tweaks can dial things in.

Ready to find your match? Browse the mountain bike range at BikesOnline US, look up the 2026 Siskiu T8 by name, and check the size chart before you pick your frame. Ride more, for less.