Ran quite a bit of 6" GRC, 6" PVC coated GRC, and 6" PVC. Wrestled big 6" three piecers together, but never thought about anything flexible over 4". Tis season was the first time I ever ran 5" seal tite for whips to Generators (mechanical vibration relief point).
For efficiency and effective installations, don't see a reason why anything over 4" (weighs in at about 80 pounds per stick) is used. I can put in 4" GRC at least twice, if not three times quicker than 6"GRC! 6" normally takes three guys to goat rope at almost 200 pounds per stick