I personally wouldn't buy 5.5" iPhone myself (especially if the resolution remains the same as smaller counterpart), but I wouldn't dismiss the validity of 5.5" iPhone market.
Although 5.5" iPhone would look ridiculous in comparison to 4.7" iPhone, many folks I know use their smartphone more as a tablet than a phone. And some folks have poor eye sight so having a larger screen makes a natural sense (larger font setting on iOS isn't the same since that comes at the expense of reduced real estate).
5.5" is a form factor that fits in a large jacket pocket or a purse and I personally would not have a problem with Apple making 4", 4.7", and 5.5" iPhone models.