Nice wheels. 
Not sure about the exact size but I’m guessing 12" so you might want to run the numbers on the KOP drivetrain transmission gear ratios to determine what your current draw per motor is and adjust accordingly: http://www.chiefdelphi.com/media/papers/3188
Ohh, your website is nice!
Thank You!
Also, the wheels are 13"x 4" and I will look into the draw per motor.