how do you match speaker size with a room?

I always had a fairly large monitor that was configured to be a floor stander if you wat that, just tilt em up and toed in. That speaker is a pair of klipsch Heresys that I have shlepped with me since I bought them as a high school student in 1976. I have used them in miniscule dorm rooms, and thru every stage in my life so far. They are now in the basement of a fairly large house after a stint in the biggish living room.
I think it is actually more a question of your taste, than anything else. Think about how different a Harbeth and a true vintage Klipsch compare. They are about the same size !! They really couldn't be more diametrically opposite in terms of their sonic signature. Add to that a low efficiency speaker like a Dyne Audio Special 25 or is it 50., which ever came out around 2002. That is the same size as well and can do both nearfield and fill a big room if you have the right power amp/s.
Do you use tubes etc. there are simply too many factors to come up with an equation of speaker enclosure volume and pleasant hippocampal massaging.
If you are really worried and don't trust your own ears you can buy so called nearfield pro monitors. I do admit that the Duntech Souveriegn is not going to really flesh out in a dorm space but even a small speaker capable of a frequency below 40Hz will take too much distance to playout the wave length. Don't think about being correct your ears will tell you if it is the right speaker for you.
The best speaker in the whole world is the one you like best of all.

Normally, speaker size is not the primary factor, but for smaller rooms, I think it is better to go with smaller speaks that will still have some room to breathe.

Here is one speaker company (www.ohmspeakers.com) where room size IS the key factor because all models in their Walsh line essentially sound similar and share the same scalable design. The drivers used in various models are sized to work best in different size rooms. The weight to the bass in a particular size room is the main sonic difference between models.