The best advice I can give here is to get all the components, and THEN decide on sizes.
I used cardboard from some shelving units I got for my wife, to mockup everything around the components I had, until I got everything to the size/shape I wanted, THEN I committed to wood.
This let me tweak marquee size, cab shape, angles, size, etc.... before anything was permanent.
Once I had everything the way I wanted it, I started cutting the plywood.
I should probably add that my cabinet base is basically a 2X4-framed rolling table.
I built the table to fit my computer, (with room for a subwoofer and coin door), put the monitor on top of it on a stand (which was actually a couple 12-packs to start with), and then stapled/taped cardboard to create the mockup of the rest of the cabinet.