The formula I use is D= .0015 SQRT(F) where D is the diameter of the pinhole in inches and F is the focal distance in mm. For a 100mm focal distance the SQRT of 100 is 10, and the pinhole diameter is .015 inches. I use an old set of spark plug gap gauges (the wore kind) to measure the pinholes. If I get below .015 I use fine wires as the gauge.
If you get below a focal distance of 100mm you will probably see some fall off at the edges of your image with the 8x10, not so much at 5x7.