I found an old racing road bike with a 50 cm frame. I would like to build it out and use it, but I am concerned it may be too small for me. I'm 5' 7.5" and I think I should be riding a 54 cm frame, but I'm not sure. Would that 4 cm difference be an issue?
I'd say "proceed with caution." The main question is whether you can get your contact points where you need them. And you probably can, but there may be compromises.
- A smaller main triangle, ridden by someone with bigger feet, will increase the likelihood of toe overlap.
- You'll obviously need to jack up the seatpost, and if you're past the limit line, you may need to get a new seatpost.
- You may need to put a longer stem on the bike in order to get the right "reach," and this can have consequences for handling. You'll probably want to put on a stem that has a fair amount of rise, to make of for the frame's smaller "stack." Otherwise you'll either need very good hip and torso flexibility or a willingness to ride with discomfort.
Also, for whatever it's worth, small bikes (such as 50 cm) often have innate geometry compromises in order to fit the frame around 700C wheels.