Assuming the larger room allows for a symmetrical presentation of the speakers, I'd go with the larger room. The downside may be that the larger room cannot be "filled" as much and is therefore more reflective, versus what you could do in terms of furniture, household items, and room treatments in the smaller room. But it sounds like that wouldn't be the case since you described it as intended as a dedicated listening space.
Floorstanders are fine in either room as long as you are seated far enough away for the sound to properly converge.
Floorstanders are fine in either room as long as you are seated far enough away for the sound to properly converge.