From the two first books (it's all I have at the moment), I take that :
The Temple has a big, 150 feet high dome above it. The dome in turn supports an 18 feet tall statue of Langhorn. It's built on top of a hill.
Coming from the Martyr's square :
A big square with a certain (unspecified) number of fountains, each with sculptures of Archangels (one each ?). The square is said to have lawns and trees, so it cannot be entirely paved.
The square must be downhill from the Temple, since there are stairs but it is not said whether there is a long walk from the square to the entrance, or only a short distance. The stairs are big enough that dozens of people can walk up and down them at the same time, and wide enough that they can step aside when Dynnys passes at the beginning of OAR.
The stairs are somehow linked to a series of sixty feet tall columns but we don't have any indication of where the columns are or whether they are supporting anything. My impression is that they are situated at the high end of the stairs, along the Temple's front. The stairs lead to a big, double, sculpted door.
Just behind the door lies the temple, or presumably (it's not clear) first an antechambre where the world map is drawn on the floor, and only then the Temple proper. The ceiling is said to be 80 feet high at the entrance and almost twice as high in the center (which is more or less coherent with the dome being 150 feet high). Since 80 feet is approximately 25m, if we imagine that the dome is an hemisphere, the Temple must be about 50 meters in diameter (and the ceiling between 25 and 50 meters high). If the dome is more ogival (does that word exist in english ?) then the diameter is smaller. A squatter dome is very improbable in my opinion, since it would not give out the right 'elevation' feeling. The altar lies in the center. The ceiling and walls are decorated with pictures of the archangels. Light comes from openings in the ceiling and is redistributed by mirrors.
It appears people can go in and out of the Temple routinely without disturbing the service or the pilgrims, so I assume there is an additional apse around the Temple itself, like those around the choir in many cathedrals. It would then be logical to expect something like columns seperating the two. If I had designed the thing, I would have had the floor sloping down towards the altar, so that the size of the building is even more impressive when in the center. The altar could then be placed on a piedestal ensuring perfect visibility from everywhere in the Temple.
Corridors branch out from the Temple (from the apse if there is one). Some must lead to public wings of the building for pastoral purposes, others to administrative wings for the different ministries, and still others to private wings where the vicars' appartments must be. These corridors are obviously real corridors, not just passageways, since they are explicitely said to have crystal-like windows.
Another room summarily described is the council chamber. It's said to be a half circular room with wooden desks and a special "something" for the grand vicar and important officials. I don't really understand the layout, but it might be similar to most parliaments' accomodation or something of the sort. Notable is the fact that it is electrically lighted.
There must be additional descriptions in subsequent books. Maybe one of Martyrs' square when it is tranformed. Maybe one of Clynthan's rooms.
I don't know if it's relevant but the symbol used on the map reminds me more of an orthodox church (which would be coherent with the dome) so maybe, more like the Kremlin than the White House, but with domes rather than bulbs ? The only problem I have with that is that I don't imagine the temple is that great a building as far as architecture is concerned. I always picture it with an 'Albert Speer' style for some reason.
