Choosing the right frame size is the single most important decision you'll make when specifying a new bike. Get it right and the bike disappears beneath you. Get it wrong and no amount of stem swapping or saddle adjustment will fully compensate. This guide walks you through the key measurements, explains how Reilly's sizing works, and points you towards the right starting point across our road and gravel ranges.
Why Frame Sizing Matters More on Titanium
Titanium frames are built to last decades. Unlike an entry-level aluminium bike you might replace in a few years, a Reilly frameset is a long-term investment — which makes getting the fit right from the outset even more important. A correctly sized titanium frame, paired with a professional bike fit, will serve you for years of riding.
The Key Measurements
Rider Height
Height is the most common starting point, but it is a rough guide only. Two riders of identical height can have very different torso lengths, arm spans, and inseam measurements — all of which affect the ideal frame geometry. Use height to identify a size range, then refine from there.
Reach and Stack
These are the two most important frame dimensions for fit:
- Stack is the vertical distance from the centre of the bottom bracket to the top of the head tube. A higher stack suits riders who prefer a more upright, comfortable position.
- Reach is the horizontal distance from the centre of the bottom bracket to the top of the head tube. A longer reach suits riders who prefer a more stretched, aggressive position.
Comparing stack and reach across sizes — rather than relying on seat tube length alone — gives you a much more accurate picture of how a frame will actually fit.
Effective Top Tube Length
The effective top tube (ETT) length gives you a sense of the overall front-to-back length of the cockpit. Combined with your chosen stem length, it determines how stretched out you'll be on the bike.
Understanding Reilly's Size Naming
Reilly frames are available in sizes from XXS through to XXL, broadly corresponding to the following rider heights:
- XXS — approx. 155–162 cm
- XS — approx. 160–167 cm
- S — approx. 165–172 cm
- M — approx. 170–177 cm
- L — approx. 175–182 cm
- XL — approx. 180–187 cm
- XXL — approx. 185–193 cm
Where sizes overlap, riders at the boundary should compare the stack and reach figures for each size and consider their preferred riding position — more aggressive riders often size down, while those prioritising comfort may size up.
How to Use the Geometry Tables
Every Reilly frameset product page includes a full geometry table. When comparing sizes, focus on these columns:
- Stack and Reach — your primary fit reference points
- Effective Top Tube — for cockpit length
- Seat Tube Length — to ensure adequate standover and saddle height range
- Head Tube Length — affects how much stack you can add or remove via spacers
Road Framesets and Complete Road Bikes
Our titanium road range is built for riders who want a fast, refined, and enduring road bike. Whether you're specifying a frameset to build up yourself or choosing a complete bike, the geometry tables on each product page will guide you to the right size.
- Road Bike Framesets — build your own with your choice of groupset and wheels
- Complete Titanium Road Bikes — fully built and ready to ride
Gravel Framesets and Complete Gravel Bikes
Our gravel range is designed for riders who want to explore beyond the tarmac without compromising on performance. Gravel geometry typically features a slightly longer wheelbase and more relaxed head angle for stability on loose surfaces.
- Gravel Bike Framesets — configure your ideal gravel build
- Complete Titanium Gravel Bikes — ready-to-ride gravel machines
Still Not Sure? Reilly offer FREE bike fit
If you are between sizes, or if you have any previous injuries or specific flexibility considerations, we recommend a bike fit. Reilly offer a free bike fit and cleat alignment for every customer. Bike fits take place at our Workshop in Biggin Hill (TN16 3AG) Book here
Try Before You Buy
Once your have got your sizing right why not demo a bike. You can demo our gravel and road bikes from the workshop in TN16 3AG or try on our indoor static trainer. Demos are available from Monday to Saturday 6am to 8pm Book your slot here
Talk to the Reilly Team
Any questions about any aspect of a bike build, just talk to Ed or Chris