I'll allow myself to throw in my thoughts really quick. There is one thing I've been always wondering about. As we know Bahamut is a primal but there is the question of was he ever a dragon (and dragon only) or born a primal and if so, does him being a dragon (and dragon only) or being a primal change his size? not to mention how he became a primal but that's too crazy for me. Who knows maybe he isn't a primal after all. Maybe a crazy lalafell was the real bahamut.
We all know who phoenix is by now and mutation/transformation/power-uppu seems not out of question.
I can't really help out with the actual size guessing though but my 2 cents.