I actually did take the bulls by the horn and developed a way to objectively choose my tarp size.
Assumptions: pouring rain, hunkered down for the day waiting for weather to pass, thus tarp needs to be staked to ground.
I measured myself sitting up to see how high the roof would need to be for my head to not rub. Measured width of my head at that height off the ground, then width of shoulders and height off the ground. Then I measured myself laying down and how much space had to be over my face so I was clausterphobic, and width and height off shoulders for comfort. Then I plotted models of myself sitting at the midline of the tarp, and two of me sleeping on backs with enough space for a child in between. I did this all in sketchup. Then I mapped an A-frame over these figures.
All that said, I came up with 10 feet wide (almost exactly), 8 feet long (a bit more arbitrary -but I am making separate beaks).
Once I had those dimensions, I then went on to sketchup multiple pitches to see if everything was still possible: lower wider pitches to get 4 people in and taller pitches when it is just myself and maybe a companion.
Overall, for purposes of multi-person utility without getting too large to make unweildy for a single individual, all all things in between, 8X10 seemed to be the ideal go-to size.