I reckon an 8x10 would possibly be easier to build, but holy cow 8x10 film is expensive. I would personally go for 4x5 with a slot to suit the normal 4x5 double-sided holders. The bonus there is that if you do it right, you could even put a rollfilm back on. Or maybe 5x7 since that's a common paper size and a reducing back for 4x5 won't look completely silly on it.

I would seriously consider making it a zoom, i.e. have a simple sliding-box arrangement (or bellows if you're extremely keen) so that you can change the position of the pinhole wrt film and therefore field of view. Of course the pinhole itself needs to be quickly replaceable to deal with size changes. And if you get that far, you've built a complete LF camera - replace the pinhole with a shuttered lens and there you go (GG for focusing optional).

Obviously you want a tripod thread in the bottom.