You should be fine. Harbeths actually seem to do pretty well in "normal size rooms," especially as they probably originally targetted British homes (I believe Alan Shaw has said so).
In any case, my Harbeth SuperHL5plus worked fine in my 13' x 15' room, in terms of even frequency balance, no bloat etc.
My only issue was a somewhat limited soundstage depth. Quite a number of other speakers produced more believable depth and dimensionality in my room (including my current Thiel speakers).
The Harbeth speakers didn't seem to have this noticeable problem in the stores, but then the store auditions I had were in a large demo room. So either they bloom in soundstage in a larger room, or perhaps it was because I was sitting further from them at the store and so due to that the soundstage seemed deeper.
FWIW....