From my 12 years experience in the construction of wooden birdhouses for Sparrows, Parus major, Parus caeruleus, Blackbirds, White Wagtails and Tawny Owl, I would have to agree with the importance of researching into the size of hole and depth of birdhouse in order to assure the suitability and success for habitation. I used the Latin names (above & linked to Wikipedia) because this website censored out the common names I originally supplied (also used by the English speaking world as well as Wikipedia!) This always trying to maintain the politically correct aspect for the Puritans is total BS.....
Anyway......I've maintained 15 birdhouses on our small property for the past 5 years and by researching and building to meet their needs, I have insured full habitation in each of these houses for the selected breeds throughout the entire year (except for the Wagtails who favor migrating to sunnier/warmer Africa for their winters).
It's very important in construction of birdhouses that are intended for actual habitation, as Mossyrock mentioned, that a means is built-in that allows for easy clean-out on an annual basis. When I clean out my birdhouses, I can visually check on the brooding season's success. Normally these birds will have three to four broods of four to six eggs during one summer season. The clean-out will show each brood level (similar to a soil stratification/profile) with an occasional abrupt end of nesting if one of the young or fledgling has died in the nest, for some reason. Occasionally I've found one or two eggs in one of the lower levels that remained unhatched, as well. After the nesting season is completed, generally the bird house is too full of material for continued nesting the following year, hence the importance of vigilant yearly clean-out/maintenance.
Our birds are most always finished with their last brood of nesting by early to mid-August. I wait until about the end of September (end of October for the Tawny Owl) to clean out the nesting boxes of old material and any insects, so that the birds will have ample time to gather fresh dried grasses, moss, etc. to rebuild their insulated winter nests, which I will witness them doing so within a couple of weeks. These birds continue to nest year round through the snowy and cold winter, while I supplement their feed with about 100 pounds of blended wild birdseed, tallow and apples. Some of our sparrows have nested continually in the same house for the past four years, so they are almost like family... Just this morning as I was taking sunflower seeds out to the feeder I noticed a little sparrow head poking out of its bird house awaiting its breakfast. No sooner had I closed the door, it and its mate were at the feeder indulging in vittles. This particular couple (pictured below during early August) has the best view of all our bird houses and are seen throughout the year just sitting together on their house, soaking in the rural ambiance. Being close and visible from the house it is also easy to keep tabs with their 'goings-on', as well.
Note the size of the hole (in the picture) compared to the size of the sparrows. The hole is 25mm in diameter and from the appearance of the birds it looks to be too small. The birds are mostly feathers and air-filled lightweight bones and squeeze in quite easily. The small hole allows for greater security towards the young.
I would add that it may be equally as important to furnish habitat during the winter, as well as the summer months, but this may be more specific to the particular region and type of birds the habitat is geared towards (ie, they being migratory or not).
My thinking with constructing a ceramic birdhouse would be to at least glaze the exterior top to provide a more secure waterproof shelter if one desires to offer winter protection as well. But as Mossyrock pointed out, the size of the hole is imperative as well as the interior dimensions, if one desires to attract nesting birds. I've found that a hole varying just a few millimeters from the specific range of size required by a distinct species of bird can and will determine whether or not the birdhouse will have borders moving in. The actual finished size of these requirements need to be taken into consideration along with the appropriate compensation for clay shrinkage through the drying and firing cycles. Myself, I haven't built any ceramic birdhouses because I always have an abundance of scrap wood to build with and wood is a better insulator from heat and cold then ceramic. My wooden houses, I believe, tend to offer cozier habitats against our cooler climate here in Scandinavia.
An additional note... Birdhouses do not need a post sticking out in front of the door for the birds to land on. I've never seen these even in the forests, where the birds find their natural nesting habitats as well. It is for the most part a detail that most avian aficionados would eliminate as it provides a perch for predators like magpies and squirrels to rest on, while feeding on eggs or young chicks.
Wow. These are wonderful responses and ideas. I guess I have some research to do too. I so appreciate the generosity of this forum. Many thanks. ~p