These speakers should always be circular, between 5-8 inches in diameter, 6.5 inch speakers being the most popular due to their moderate 8 inch total diameter of the speaker with bezel. In ceiling speakers are often the obvious choice of speakers due to the ceiling having a great deal of space which has no art. The placement is still critical to achieve the best sound in a room. Try to keep the speakers away from ceiling fans as they can cause a chopping of the sound which is very annoying. Generally, in the middle of the room in line with the lights or the heat registers is good placement. When faced with the option of closer together or further apart keep them closer together because the listener can hear both speakers because half of the stereo mix is not nearly as good as mono. This is why many manufacturers have gone to dual voice coil speakers which give stereo sound from each unit. Using one of these in a small room or using 2-3 in a room or outdoor patio can be much better.