I generally follow the Audubon building plan: 6" x 6" interior (or thereabouts) with a 1-1/8" to 1-1/4" diameter hole six inches on center above the floor, with a few inches of headroom above that. I've built dozens and dozens of them. Most of them I've given away.
Usually they are occupied by Common House Finch, Common Sparrow, or Black-Capped Chickadees.
I used to build smaller ones for the Swallows up at the cabin, but Swallows won't nest unless there's water right nearby so that's a waste of time here.
The owl boxes and bat boxes I've built were for other locations.
A couple of the more interesting ones were a pair I built out of wood salvaged from an ancient barn - I duplicated the gambrel-type roof of the barn on the birdhouses.
Another batch I built out of driftwood pieces I've gathered off the beach. Interesting challenge. Requires a lot of fasteners and imagination.