I'll leave the size to you, but Ole is right about online film buying. There is very little to choose from here in Tucson, a city of 1/2 million people, but a wealth of choices on line, so that is where I shop for sheet film.

Think about how you shoot, landscape, city, architecture, abstract, people? How much stuff can you lug around comfortably to take the shots you want and how far are you willing to walk with that much weight? Good luck on your project. tim