One thing you might try, Alex and James, to get an idea of how it will impact your backyard size and your backyard lifestyle is to cut 4 pieces of wood (or use 4 stakes and run a string) of the footprint of the size greenhouse you would like to have and where you want to locate it, and live with it for a week or so to get a better idea whether a greenhouse will cramp your backyard and style.
James, as far as materials and size, I don't think (IMHO) there are any "best" for a standard backyard hobby greenhouse. It's mainly a question of affordability and preference.