double 6" or 7", per side.
Fast, clean. mates up better with a tweeter.
MTM arrangement (midbass-tweeter-midbass)
with the right crossover, pointed straight out, you at the tip of the triangle between them. if done right, the levels will be fine. Then a window of toe in is available, of maybe 15 degrees. quasi 18 on the tweeter, quasi 12 on the woofers.
Tweeter must be a monster. King of the hill in bottom range, power handling, and efficiency. The Holy Trifecta of Tweeters. MDT33 or the ET338, is a good choice. Crossover in the 2.2k range. Some ribbons might work. Probably don't have the efficiency, though. You generally want a good 91db for one watt on the tweeter, preferably 92-93db efficiency. You'll be doing phase and crossover tuning and that will eat efficiency up, so you need to start with overhead.
be at least 9 feet away, for the pair of woofers to integrate better, and be more like a point source, than a line source.
But then again, anything can be tweaked.