Optimal speaker size for given room, or drivers?

The questions you've asked are very complex and would require a book (more likely several books) to address properly, not a few paragraphs in this forum.

What further complicates it is that you may change to a smaller room in the future but you're not sure when and you have no idea what size the other room will be.

A speaker with strong and extended bass performance might work well in your present room but may totally overwhelm a smaller room and sound terribly unbalanced.

If I were in your shoes, I'd concentrate on getting good sound in my present room and if I had to sell the speakers in the future to get good sound in a different room then that's what I would do.

How many drivers a speaker has is probably not that relevant to your particular situation. Anything from a single-driver speaker to a multi-driver line array could work well in your room... depending on your listening biases, associated gear, aspects of room acoustics, etc.

In other words your questions are far too vague to be answered with any certainty. Perhaps others could suggest some informational links for you. My view is that there's no substitute for a little trial and error. At least that way, your preferences will be based on your own experience and not somebody's opinion which may or may not represent your needs.

And besides... rooms are almost like fingerprints... each one has its own acoustic wrinkles.

Maybe you should simply buy decent but robust monitor speakers (or any decent speakers) that don't have significant output below 80Hz. Then you could fill in with a subwoofer or two and only change the subwoofer(s) if necessary, when you move.

To be more specific would be like a blind man trying to discribe a scene he's never seen.

Your current listening room size is very close to mine. If you want a full range speaker that could work well in your current room and still be appropriate for a possible future smaller room, I recommend you consider the Vandersteen Wood Quatro's. They are exceptional speakers that are in your apparent approximate price range, and have the advantage of being tunable to an individual listening room's "needs." They were the speakers I was going to purchase until I heard the Sonus Fabers. It was a tough choice, but for me, the Sonus Fabers won out overall in terms of musicality and involvement. However, if I thought I would have needed to place the speakers in a significantly smaller room in the future, I probably would have purchased the Quatro's. Although I found them to be a bit less musical and involving than the Sonus Fabers, I have never heard a speaker that images better than the Quatro's.

Best you find a dealer that will let you borrow demo speakers, otherwise it is a crap shoot. Nothing, and I mean nothing is a substitute for listening in your environment with your equipment driving them, and your favorite music playing. Acoustic wrinkles in your listening space, as noted, will play havoc with 'buying on paper', or the dealer's room. Dealer demo's are typically 'broken in' and often they come with cables that the dealer has found make them shine. In over five decades of being involved in audio, I have never purchased a speaker that was not provided as a demo to try in my 'chain/enviroment'. Interestingly, in most cases I purchased the demo's at a nice discount. Two weeks should be the amount of time needed for you to make a judgement. In these economic times, dealers are going the extra mile to make sales, so take advantage of it. Hey, with gas at $1.50 you can make 'sleping' pay.