Karen,
In reality they are not that big. For instance my #2 Singer 66 treadle cabinet is only 18" deep X 36" wide. There are smaller ones too. Of course when you open it you need to have room for the hinged lid. One treadle doesn't take up much room.
The problem comes in when you have 2 treadles and 4 cabinet machines in a small room. Things get a bit cramped then.
Joe