If you buy a 2.5" diameter cylinder your cock will expand in height which will allow you to add lots of girth but if you want temporary crazy girth increases (lymph fluid swelling) go for a 2.75" tube and build up slowly until pumping for several hours is comfortable. Careful about pumping at high vacuum pressures, you can get blisters and discoloration - which can take up to a year to disappear.
Buying a cylinder that is too big can cause your balls to be sucked into the tube which can hurt, so don't go for a 3" diameter until you pack the 2.5" and then the 2.75" cylinders.