I bought the mityvac hand pump some time back, and the aquarium gravel cleaner tube which is 1 7/8" or slightly larger ID. My erect girth at base is 5.5" or a hair over, and the tube seems to be the right size for now. I've not done many sessions yet, but so far I have yet to see a significant "pumped" temporary increase. I do see the veins pop out much more while in the tube, but that's about it. I get the idea that most people think you can pump up to a noticably bigger size, but so far I've not been able to do it.
Likewise, I've read many times about going in the tube flacid and "pumping up" to an erection, but that hasn't worked for me either. I've only used it a few times, so I'll keep trying just wondered if anyone else had similar results?