Good to see you here, asking good questions!
I’ll chime in on the build size. I always recommend making it as short as possible, it doesn’t sound like you are interested in 3d printing with it so make it really short. that gives the biggest rigidity improvement for aluminum and better cutting speeds overall. After that make it just a bit bigger than the biggest piece you would use in the X and Y direction. It really isn’t that big of a deal to go bigger if you need to if you plan ahead a bit and leave extra belt and wire on each axis.
My vote is for small!