Originally Posted by johngwheeler

Good advice; thanks! I think getting a bike that isn't already at the limits in either direction (too small or too big) is definitely to be recommended, just to have some latitude for minor adjustment. There is probably less adjustment in practice than my 5-6cm guesstimate. More like 2cm with stem changes and spacers.

However there can easily be 2-3cm variation in frame geometry for the same nominal frame size from different manufacturers, so it seems really important to have an idea of what geometry dimensions will work for the individual.

My personal experience is a bit limited, but illustrative. My first bike (Trek Crossrip 54cm) seemed just about right (I had tested a 56cm and 54cm, and both were OK, but the 54cm just felt a bit less stretched out and more controllable). My second bike (Giant TCX) was a medium, recommended by the LBS, and in-line with Giant's sizing charts for my height (177cm). However, I think I probably would have been better choosing medium/large because I have long legs for my height, and prefer to have less saddle-to-bar drop. My current test bike is a Fuji Gran Fondo (large, I think, but there are no labels on the frame) - this is definitely a bit too big, but may be correctable with a shorter stem.

I guess I got lucky with the Trek, and had a better experience at the LBS who had both sizes and let me try them. The Giant dealership only had the medium in stock, so this is what they wanted to sell!

I had a bike fit on the medium-sized Giant, and between this and my Trek, I probably have enough data to determine my optimal frame size. There is of course no guarantee that these will coincide with bikes that I am interested in, which is why I asked the question about the amount of available adjustment.

Thanks!

John