Originally Posted by
spectastic
ok. since the saddle is a direct extension of the seat tube and the stem is a direct extension of the head tube, it really shouldn't matter what size as long as it's within reasonable range, right? so if you want to go from 54 cm to 58 cm, just raise the saddle by 4 cm, which isn't a crazy change at all
Not necessarily. A lot of riders use effective top tube as the most important measurement as opposed to seat tube length because of reach. If you take the geometry chart below as an example and compare for sake of argument the 51 & 56cm frames, the effective top tube is 54cm on the 51 and 57.2cm on the 56. Just raising the saddle 5cm does not change the frame size. The effective top tube distance still stays the same. So if somebody should be riding a bike with a combined reach of say 68-70cm (effective top tube + stem length) it's going to be somewhat difficult to obtain that with the 51 because as Drew Eckhardt pointed out 140mm stems are the longest that are readily available, although in this case that reach could be achieved but there would also be a good amount of saddle to bar drop on the 51, which is not always preferred for a commuter.
Anyway, bikes are made to be adjustable, so my opinion is ride what works for you
