Hornet was around 75cm from wheel to wheel and the disc would still cause it to ride around on one wheel turning a corner. The disc on that was only 1.8kg as well. I believe that one is closer to the 3kg mark in which case I'd highly recommend a wide wheel base otherwise it will just flip itself over again and again.

Are you going to make it invertible? As it stands the disc would be smacking the ground on it's back.