I suspect that people won't be buying a projector screen to use with the Pico. Rather, they will project onto a white or near white wall. It's been a long time since I've seen one, but mini-screens do exist, including those that retract into a tube. I've seen such small screens in sizes from about 16" diagonal. They can be opened up, and set on a table. By the way, I recommend finding a mini-tripod for setting up the projector. Those weigh only a few ounces and are typically less than 6 inches long.
If you are going to get a screen, I'd suggest a high gain screen if possible. That will help with ambient light, and provide some extra brightness. Some mini-screens come in high gain surfaces. One example is the Da-Lite Presenter screen shown here. It is 40" diagonal, sells for around $199, and has a particularly bright surface.
For a lot less money (around $50), Draper makes a 25" diagonal tabletop screen for pico projectors. I can't say that I've actually seen one, but here's an image I found.
I'll be at Infocomm in June, and look around to see if I can identify more suitable portable screens.