Well, my rock hard erection (mid-shaft) width is a bit over 5 cm so basically I'm packing that tube and the base is quite tight. I'm not trying to pack like those hardcore pumpers, since I'm just cycling the blood in and out without that much fluid build-up under my skin. I'm soon having a new tube that is only a little bigger in width (~6,5 cm inner width and ~24 cm length) since too wide tube is a nightmare.
My pistol pump can handle the water so I recommed to find a high quality pistol pump. I use Fröhle Gmbh -pistol pump that gives 800 millibar suction that equals ~23,5 inHg. Of course I use that maximum level only for a short time at the end of my session, but it actually shows how comfortable water pumping is, since I feel no pain at all. Before I used much weaker pumps and air-only method and my penis was usually hurt, bloated with fluid and the skin was filled with red and blue spots.