IMHO, 4x5 is the way to go for this. As I mentioned earlier, it is not all that extreme of an enlargement with today's films. Blowing 4.5 inches up to 60 or 72 inches will be a 13x or 16x enlargement, respectively. It's a lot of enlargement, but not unheard of. The 72 inch print will be roughly equivalent in quality to filling a piece of 16x20 paper from a 35mm negative. It starts to fall apart, but IMHO it looks good. It will be important to compose in camera such that you do not crop much when printing, however.
I'd take a look at T-Max 400 for this project. It's an outstanding film that will help you to minimize grain.