It also depends on what you're planning to make it from. If you plan on building the whole thing from Hardox, you'll have to make it smaller than if you were planning to use aluminium or HDPE. A lot of Hardox FWs I've encountered seem to be around 350mm long and wide, whereas my HDPE bots are around 50% bigger.

Also, bear in mind whether you want to enclose the wheels or not. Putting the wheels on the outside leaves them more exposed, but it means you can make the chassis narrower.