I've been considering the same here is what I have for 8x10.

Min Focal Length is 7 inches with optimal pinhole at .56mm - f=317
the fstop rises with increased focal length from 317 at 7in to 412 at 12 inches.

Consider building at 9 inches with a 2 inch reversable lensboard as an accessory. Then you have 7,9,11 inch focal lengths so you can choose the best angle of view.

F Pinhole Angle Fstop
7in .56mm 85 317
9in .64mm 71 357 (8 stops > f22) (iso 100 sunny 5seconds)
11in .70mm 60 400