I can't really answer your questions re max size or heat retention but i'll give you my opinion. We have some roman blinds, and we had them fitted outside the window - i think they look much better fitted outside rather than inside the window, it frames the window much better, but that's just my personal opinion.
I don't have any windows as wide as yours but i recently saw a window as wide as that with roman blind, and i would recommend that you make sure your blind is interlined so that it has a stiff feel to it, that way it won't sag as much in the middle of the window, and it keeps the folds/pleats looking well.