Step 1: Determine the size
What I will point out is that you want to consider your building materials as you pick a screen size. Lumber and paneling is easiest to get in 8' lengths and king size sheets are only good for about 100" wide. Given the building material factors I sized down from an initial 100" wide to 96" wide. That's still 110" diag for a 16:9 screen so it's plenty large. Not only that, but the trim around the frame will add about 5" to the width, leaving me with just a couple of inches of width left on each side in my space.