1.7i vs 3.5r

You are comparing apples to oranges, i.e. quasi ribbon (1.7) vs real ribbon. But correct me if I am wrong.

I've had both 1.7s and IIIs. I'd go with the 3.5, assuming you have a decent size room. Maggies's true ribbon is magical.

+ on needing the "decent room size", and of course, an amp with some guts. You often see Maggies crammed into rooms that way too small for them to work properly.

If you have the room and sufficient amplification, 3.6r would be a great affordable choice.

So what are we talking with respect to room size with the 3.5r and also is ceiling height an issue?

IMO, celing height matters no matter what the speaker. But I've mostly had 8ft ceilings and no issues.

I have my 3.7i in a 12 x 15 room and they sound amazing. I previously had 1.7i and these just sound so much better. As long as you can get the 3 ft  between them and the wall you should be good to go.