Minimum Sound Bar Height

My wife and I just moved into a summer home and I want to set up a 5.1 system for watching movies.  A top-quality system is out of the question for this place; reasonably high-quality consumer-grade level (preferably, used) will have to do.  I plan to hang the TV over the fireplace using a MantelMount, which also hangs the center channel speaker.  Since the bottom of the speaker must clear the top of the mantle, I need its dimensions before I can mount the TV on the wall.

A low profile speaker would have some advantages but I question whether small drivers can produce even reasonably good quality.  I'd love to hear opinions re how tall a sound bar needs to be to produce decent sound, or if this line of reasoning is completely wrong.  (The mains will be a pair of old Magneplanar MG-I after I re-cement the tweeter voice coils, which occasionally rattle a bit.)  Does anyone have a specific suggestion for a center channel speaker that I might find used?
Bob Simon