You can go pretty small if you need to. I'm set up on a small bench on one side of my workshop.
I tend to spread out and work on different benches for different stuff (trim cases over there, run the tumbler over there, sort brass in front of the TV in the living room), but I could function with about 18" wide by 12" deep bench space to mount the press and the powder measure with enough room for the reloading block in between. Powder scale goes on the wall on a little shelf at eye level. My powder and primers sit in a styrofoam cooler on the floor in the corner (the shop isn't air conditioned), and die boxes and whatnot go on a small shelf or in a tackle box at the back of the bench.
When I move next month, I'm getting a smaller workshop which is my trade-off for a bigger house so I'll probably mount the press and powder measure on a 2x6 that can be clamped to a workbench and put away when not in use to free up the bench.
Basically, your reloading gear will swell to fill whatever space you let it take up. If you have no space, you can do the job with a hand press and some counter space for a powder measure and scale. If you have a barn, you can probably fill it with reloading crap if you put your mind to it.
-J.