It's a little more complicated than that. If you're looking to get 350mm usable usable working width in the Y direction the board has to move 350mm both sides of center. At a minimum you'll need 700mm of box depth. But that's just based on board movement. Any excess the board sticks out beyond the usable area (ie. clamping area) and any amount the build sticks out beyond the max range of the board also gets added to the 700mm.

Truly the best way to keep the system to the minimum form factor such that it can be put in a box is going with a fixed bed system like you already have.