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.
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.