How big can they be overall? First thing that comes to mind is the SB Acoustics large format ring radiator at madisound. But you need to have the space to fit it.
I'm actually lookin for some small ones around 25 to 28mm because I'm replacing some ht25 that I recently blew. I'll either be running them crossed passively in my comp system that gets 190w. Or maybe active with about 50 or 80 depending on if I decide to get another amp.