condom/water pumping cylinder size

Ball sucking issues aside, assuming a person was engaged exclusively in condom or water pumping would the diameter of the cylinder really be an issue? Obviously it would need to be big enough for the dick to fit inside, but apart from that it seems to me that the diameter would be made irrelevant because of the pressure from the condom/water.

Dream

Would be, if the water and the condom were always keeping you from squooshing yourself badly up against the cylinder walls.