I tried building bluebird houses over the winter as we have many down here. Mr.Bluebird must be carrying a tape measure with him because all are unoccupied.
We have several different Bluebird houses, and the one that they choose most commonly is this Peterson style house.
https://www.dutchcrafters.com/Amish-Poly-Peterson-Blue-Bird-House/p/54509
Tips for building your own Peterson style here. https://feltmagnet.com/crafts/bluebird-house-slant-front
They’ve also built nests in this one every now and again.
https://www.lowes.com/pd/10-875-in-H-Cedar-Wood-Variety-Nesting-Box-Bird-House/1000184213
This one is pretty similar to the third one that they’ve used sometimes.
We also have one on a post that we bought from Wilds Birds Unlimited, but the House Sparrows fight for that one, so the bluebirds never choose it. I keep it regularly cleaned out so the sparrows never lay eggs in it.
More tips on where to place the bluebird houses here.
https://bluebirdlandlord.com/where-to-place-a-bluebird-house-the-best-location/
In my yard, the one they pick the most often faces a little bit southeast, and the others, variations of southeast. They only choose one box at a time. I think because mine are too close together.
Our yard is heavily wooded, so these are placed with the woods behind them and the lawn area open in front of them. I can see all of them from our windows, which makes it convenient to see when they begin feeding the babies. Their mealworm feeders are located near out deck, too, so I can refill those quickly and easily. Hope any of this helps.