Well if you're just stretching it over a small wooden frame, I would suggest getting 3-4" extra on each side so you have a little wiggle room and enough to properly attach it to the frame.
That means 71.6" x 109.76" for 120" diagonal 16:10 (including the 8" extra)
That means 87.5" x 135.2" for a 150" diagonal 16:10 (including the 8" extra)
If you're making a masking system and border for the screen, depending on your mounting option of choice, you may want to adjust the aspect ratio of the screen a little bit to have a constant size around the edge that will be covered up by the boarder... or you can just cover it up in the right ratio of 16:10 when you end up overlapping the screen with the border so you still end up with a slightly smaller (but still correct aspect ratio) image.