you see a basic concept, 4,5,6,7,8,10,12,15 inch.....in the stores and webstores
then the standard 3x3ft triangle, as well explained in Ethan's website....
the monitors targeted at HR seem to be 4 to 8inch, average bedroom sized mix room can use as much help in acoustics and panels and traps as you can afford.
6inch & 7inch seems to be a popular middle ground, in general. look at Adams and DynAudio .. yadayadayada
5",4" some like, some need the 8" bass .... not many use 10" or 12" or 15" in the 3ftx3ft triangle, though some use PA speakers for mixing...