Specifically, to address your concerns about the torsional strength of varying shaft sizes, I have found a couple of references that might help you. First, at the bottom of this page (
http://www.ecs.umass.edu/mie/labs/md...r/statload.htm) is a calculator that will give you the torsional stress for a beam of any cross section. You will need to input the torque applied, the distance of the "fiber from the neutral axis" (the shaft radius), and the polar moment of inertia of the section (a formula/definition/discussion of "moment of inertia" for a circle can be found here:
http://www.efunda.com/math/areas/Mom....cfm?Moment=zc).
Then you need to know the strength of your material, and what type of safety factor you want to apply ... and don't forget to keep track of your units ...
I am sure others can elaborate more ...
Good luck!!
edit: Sorry ... I did not realize that efunda site required membership, however I believe you can sign up for a limited free membership to access this information.