smallish plants that have been trained a little are about 18 inches across. Plant numbers are unimportant for yield to some extent. What is important is primarily how full you can make your canopy, and secondly how quickly you can get to this point. That is where plant numbers come in with regs. The faster you want to flip those, the more you need.
With autos, you are going to need to grow one out, while taking notes, so you can use the info to calculate the best way of filling the canopy, as you can't control it with light.