First and foremost - you are talking about some ultimately portable projectors. Will you want to even be carrying a screen?
You can now buy "portable" screens. Elite Screens portable screen for pico projectors for example, is only about one size up in size, compared to a normal sheet of paper (and less than an inch thick when folded). It is a very suitable screen for small and bright presentations, gaming, or, probably what you have in mind, if what you need is a 10 inch diagonal lightweight screen, that fits easily into a briefcase or... There are a variety of small screens out, but finding them might be the biggest problem. If you are working with larger, screens, though, and are portable, you'll probably perfer a white surface with at least some positive gain. Those with a lot of gain will limit the viewing angle, as a trade-off for brightness. Still, you can almost double your effective brightness. (That might be good for one man gaming, but not so hot for a presentation with people sitting in a wide arc).