Speaker height vs ceiling height

The size of the cabinet is not the issue; it is the position of the driver within the cabinet relative to the boundary that causes a concern.

Radiation patterns are effected by room boundaries. So radiation pattern should be considered when near ceiling etc. A dipole with figure 8 radiation would work quite well as would line arrays or horns designs.

I would think you want to avoid early reflections of ceiling similar to walls. Usually there is less distance to work with in regards to ceiling and floor than walls even.

I like to keep my speakers as low to the ground as possible but also with adequate elevation from floor. Having drivers as close to listening position elevation or slightly below is probably a good practice in general.

Tall line arrays might be a different story, but good ones that tall will cost a fortune.

Vertical dispersion in lines planars and stats work often in lower rooms. From what my ears say atleast.

Map,
You sure got that right! I heard Audience speakers...the 2+2 at THE in Newport a couple months ago.
5 large, but really special.....

Right in back was the line array of 16 (count 'em.....16) idential drivers. I didn't bother to ask