I found a 48" square window screen kit and a roll of 48" screen material at my local hardware store, so that's the size I went with for my selfie spot. This is a pretty good size for a one-person selfie. The great thing about these screen kits is that you can size them up or down really easily, so measure your space and pick your size accordingly.
Be sure to grab one of the screen installation tools as well. It will make your life easier!
Even though my frame was 48x48, and my screen material roll was 48" wide, I still needed to trim 1" off my frame, because installation requires a bit of overlap with the frame and the 48" screen fabric didn't quite reach. Use a hack saw or a chop saw or whatever you have to get a clean cut.
Use a flat head screwdriver to pry out the foam rubber spline in the frame's groove. Pull it out completely and set it aside.
Cut your screen to size, leaving about 1" overlap on all 4 sides. I found it helpful to clamp it on all 4 corners.
Use the convex side of your screen tool to tuck the edge of the screen into the groove. Place the foam rubber spline on top and use the concave side to firmly push the spline down over the screen.