I would love a 5.5" iPhone. However, I have the Note 3 (5.7") and my 8" tablet is still way bigger than my phone. Both can easily coexist.
dependant on the IOS ecosystem to switch so I came back to the iPhone but that screen size was something else. I don't think that I would ditch my ipad mini but I probably wouldn't carry it outside of the house that much any more.My use case for my ipad is situations in which an iPhone screen is too small but a laptop isn't portable enough. That's not many situations, and a 5.5" iPhone would be "big enough" more often than a 4" iPhone. I think I'd keep my current ipad but not replace it in the future.
Of course, as others have said, it would depend on whether a 5.5" phone could work for me at all, and I doubt it could (due to not fitting in a pants pocket).
the pants pocket is a good point, back when I had a note 3 it was an issue. My 3 kids have severe allergies so I always carry 3 epipen with me so I have a messenger bag but if you don't it's an issue. There are belt clips but its still big.
A belt clip would be a deal breaker for me. I don't know way, but I just greatly prefer to carry my phone in a pocket.