Why Brand Sizing Differs: Giant vs Merida vs Orbea Geometry Compared

By Mike · Updated 10 August 2026

A rider walks in, says "I am a 54," and three bikes on our floor politely disagree with each other about what that means. We stock Giant, Merida and Orbea side by side, so we live this daily: the same body, three brands, and often two or three different "correct" sizes. Here is why, and how to navigate it.

Why sizing was never standardised

A frame size number originally described a seat tube length, but modern sloping frames made seat tubes a design choice rather than a fit measurement, and every brand slopes differently. What actually determines fit is where the frame puts the handlebars relative to the pedals, described by stack (height) and reach (length), as unpacked in our sizing guide. Two frames can share a label and differ meaningfully in both numbers, or share the numbers under different labels. The label is marketing; the chart is the truth.

How each of our brands approaches it

Giant committed hardest to compact geometry and sizes its road bikes S, M, L rather than centimetres. The upside is a design with generous standover and long seatposts; the practical effect is that riders converting from numbered sizing often land on a label smaller than instinct suggests, and the honest conversion is via the geometry chart, not the letter.

Merida uses more conventional sizing across its range, with distinct race and endurance platforms that differ in stack and reach at the same nominal size, so the model decides as much as the number does.

Orbea likewise sizes conventionally, with its own ideas about proportions through the range, and its race and endurance platforms split the same way. Orbea's custom-build options also mean two examples of the same model can wear different cockpits, which changes the fit as delivered.

What this means when you shop

Three habits keep you out of trouble. First, never carry a size between brands: being a 56 in one tells you your neighbourhood, not your address, in another. Second, compare stack and reach between any bike you own that fits and any bike you are considering; five minutes with two geometry charts prevents the classic wrong-size purchase. Third, when you are between sizes in a given brand, and across three brands you will be between sizes in at least one, the tiebreakers in our between-sizes guide apply.

This matters double on the used market, where a listing's "size 54" might be any of three different fits depending on brand and era, and there is no exchanging the frame afterwards. It is exactly why the used bike buyer's guide puts sizing at the top of the checklist.

The shortcut

Get your coordinates measured once, properly, and every brand's chart becomes readable at a glance. We do this as part of a sizing assessment or full fit in Cheltenham, and because all three brands are on our floor along with a rotating pre-owned range, the theory can be tested the good way: back to back, on your body, in one visit.