If you know all the parts that are going in the machine, make some 1:1 scale cardboard models of every part and then sit down and try and make them go into the smallest space possible. Try various configurations until you get the size right down.

You obviously need to keep in mind where wires will run, where your wheels go and where the chassis will be around all the parts as you do this. I managed to shrink the volume of my new machine by about 30% by just sitting down with all the parts for a few hours and trying different layouts.

Put time into the planning. It will save you money, weight and time in the future!