Luna-Tic measures 400mm wide and 500 long (550mm including the lifting scoop).

Stumped me to stat with as well. What I did was start with what I knew. I'm using cordless drill motors that are 150mm long and they are end to end either side of the bot, allow 50mm between them for space, that's 350mm. The wheels screw directly to the motor, so no added width there, finally adding the thickness of the side armour with a little space for the wheel to wobble about in, comes out as 400mm.

To work out the length, as I'm using 4WD skid steer, I wanted the wheels to be square to each other (the same distance apart sideways as they are long ways). Adding a little wedge on the front of the chassis itself and the scoop, comes to as it is. Its maybe a little on the larger size, compared to a two wheeled bot but it has a nice low profile and plenty of pushing power. (I hope, not combat tested yet) ;-)

There's no wrong size, but make sure you can fit everything inside. Weight is the major issue of concern, weigh everything as you fit it and keep track of the 13.6kg allowance. So if you can make it 50mm smaller do, that will save you weight in the long run regarding chassis and armour.