There is a few steps to building a chassis, I would recommend looking at this article Catalogue of Drive Gearings to pick a wheel size and gear ratio first. Then once you find a ratio you like, you can plan out wheel spacing, and final length.
To get your drive speed, multiply the RPM • ∏ • wheel diamiter/60 = in/second
I am personally running 450rpm on 2.75 this year, but pick what works for you