Some basic Physical Principals:

Calculate robot speed:
(pi*D*RPM*60) / 100 000 = robot speed in km/h
Where: pi=3.14 . . .

Calculate required battery size:
This is often no easy feat, as there are many variables that come into play, and every fight is different.
It is generally easier just to investigate what others with similar drive line components are using.
However, it is possible to obtain a reasonably accurate result using the relevant infomation at Paul Hills
A technical guide to fighting robots" http://homepages.which.net/~paul.hills/Batteries/Batteries.html∞
To obtain a more accurate figure, EDTSim gives better results, based on your own drive train setup, and also
includes a lots of other performance data.

Calculate energy of a spinning mass:
Infomation @ Paul Hills website http://homepages.which.net/~paul.hills/Spinningdisks/Disks.html

Or if you don't like maths, the calculator will do most of the calculations for you: http://www.teamcosmos.com/ke/ke.shtml∞