I just finished installing speakers for my whole house audio system. For larger rooms > 14x14, I installed at
8" ceiling speakers. In smaller bedrooms, I used
6.5" ceiling speakers. In great rooms, I installed multiple 8" speakers.
For the master and family rooms, I'm using the 8" ceiling speakers for whole house audio and then using a
automated speaker switch to change them to rear surrounds when using the 5.1 receiver for TV viewing. Also, for these rooms I installed
8" in-wall speakers for L and R channel for the 5.1 receiver. I didn't install in-wall center channel speakers since I don't want to commit to their placement. I can use some voice matched
bookshelf speakers for placement and furniture flexibility.
I'm not installing the in-wall speakers until I move in to make sure the placement works for the furniture and ultimate listening positions, but the speaker manufacture recommends that the tweeter should be around ear height. You also don't want to put the speaker to close or too far away from the corners. At least 1/3rd the distance from the corner to the midpoint of the wall was a rule of thumb I heard (if that made sense).
Not too hijack this thread and make it a whole house audio discussion, but the speaker performance is highly dependent on the amp used. For my great rooms where I use multiple speakers per zone, I'm using a
multi channel amp. Even though this amp is not high-end, it pushes the sound out of the speakers (esp. the bass) far better than the remote
in-wall amps I use for zones in smaller rooms. The multi-channel amp gave me that gitty feeling that I haven't had from listening to music since I owned a portable DJ system in high school.
I would include a provision for a sub-woofer if you want to have solid low end response to music listening or if you plan to use the location for home theater.
Finally, when you install the in-wall or in-ceiling speakers add some fiberglass insulation in the stud space, although don't stuff the insulation.