I typically layout the rough design of my extruder centred to the build volume of the printer. I then design my gantry and other components around that, and then lastly the frame. This also gives you a good idea of where motors, rails etc will end up, and allow you to choose how/what you want to build the frame.

Frankly, through a dozen designs or so already, designing a printer around a frame/set size just leads to too many compromises in the actual bits that matter. Design a good solid gantry and bed first, and worry about the frame and foot print later