Argus if you're making it yourself then I saw screw 4x5. Screw 8x10 to. No reason to limit yourself to the formats others invented and liked. If I was building one for myself the choice might be 10x20. Okay hard to find lenses. 10x15? Certainly bigger then 8x10.

In many ways 5x7 is the perfect format. The camera isn't much bigger then a 4x5 so you can hang a 4x5 back off it not feel like you're lugging a lunk of extra weight. Many 4x5 lenses cover 5x7 so lenses cost less and can do double duty. It's big enough to contact print.

Placing a reducing back on an 8x10 might make sense in the studio but out in the field? Even a light weight 8x10 is heavier then a 4x5 and many 5x7. Odds are the 8x10 won't handle short lenses very well. The chances an 8x10 will handle real short 4x5 lenses is pretty low.

You should be able to order 5x7 or 13x18 film easily enough. Both will fit a 5x7 camera but need different holders.