Chris, My shop is 560 sq ft (20 x 28) and I can squeeze a cabinet job out of it, but it takes planning and a lot of moving things around during the build. I've recently turned down some cab jobs just because I don't want to deal with the lack of space. Does not answer your question but here are a few other tidbits.
I'd go with a min of 10' ceilings. You want to have a clear height of at least 8.5' so leave space to hang lights, ducts, etc...
Adding a couple rooms (finishing, storage, bathroom) gives you more wall space. I ran out of wall space as well as floor space, so design any additional room knowing the walls will be very useful for storage.
Wish I had running water (sink, commode) in my shop.
Lumber and sheet good storage will be critical. When designing my shop, I planned on storing sheet goods leaning against a wall, stacked 4' high x 8' long. Placed outlets @ 54" off floor just to accommodate this. But in reality, it was way too hard to pull a sheet from the back, or even the middle of the stack. Ended up storing sheet goods perpendicular to the wall standing up 8' tall. I can slide any sheet needed out way easier than digging behind a stack. I'd recommend trying to incorporate sheet good storage in with any additional room you build. For example, one end wall of a storage/finnishing room could also be a "leaning post" surface for vertically stored plywood.
Excited for you. Good luck.