You are overthinking this with this, size room 20 watts will work with nearly any loudspeaker.
Even with a low efficiency monitor 20 watts will get you plenty loud enough
Look at a pair of Elac carinas 85db sensitivity will be loud at 85db the roon is very small and 85db is plenty loud for such a small room.
so you can look at nearly any loudspeaker.
I would be more concerned, with impedance and current deliver, however most 2 way compact, and affordable, monitors will be easy to drive.
Dave and Troy
Audio intellect NJ
Elac, Legacy, Kef Atc , Dali, Cabasse, dealers