Thanks everybody. I've been pondering over the design for a while, and I've decided that want a sturdy machine, as small as I can get it, I had a look at ratios and by the size of my wheels the rest of the chassis should come to around 400mm*256mm! My current problem is I can't fit the drill motors onto the wheels, I've thought of two ways to fix it, the first being my preferred of having the motors parallel to the wheels, but slightly down a bit, and attaching it to the axel on the wheels using bevel gears and of course the other which is to simply expand the machine - which I currently don't want to do for a few reasons: the larger it gets the more surface area I have to cover with armour increasing weight; I'll have to put more support onto the frame and the fact that I expand it means I'm going to have a large amount of empty space, which I want to prevent. So which one would have a larger advantage? :O
My design so far is here: EDIT: IT'S SO BIG HOW DO I MAKE IT SMALLER