I'm calculating where exactly to put the projector shelf on the back wall of the upstairs lounge in order to create the best movie-viewing experience from the couch. I used the table in the back of my projector's manual to figure out what the projector is capable of, and was able to determine the range of screen sizes I could have.

While looking online to see what standard screen sizes are available, I came across this Viewing Distance Calculator".

You can use this not only to determine screen sizes for projectors, but also for TVs. It's particularly useful for TVs, since it tells you the optimal diagonal measurement for both 4:3 ratio and 16:9 ratio screens instead of just the width of the image. This is much easier than using the Pythagorean theorem and backing into the diagonal (what I had to do when I was shopping for a TV nine years ago).

The website also gives a range of sizes based on various certifications. Two film technical organizations, SMPTE and THX, set standards for the viewing angle necessary for movie theaters to be certified by them. So if you want to set up your home theater to professional standards, you can see what they would be.