I suspect that people won't be buying a projector screen to use with the P4X pico projector . 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.
You can probably find some place to project a smaller image, when you need to. Almost anything will do, as long as it's pretty white. There are plenty of small screens out there, all you have to do is figure out what size you need, what you need it for, and you should find screens at the same AV type dealer you buy the projector from. If you are shopping more generally for your pico projector, some retailers and online sellers won't have a clue, but don't let that discourage you. There are a number of choices. The real question is, are you needing a screen you can travel with.
Note, many of these screens will have some gain, (add brightness) and that also allows some minor rejection of side ambient light. Basically, therefore, a screen like that is likely to give you a brighter image than your living room wall.