Harbeth Hl5 plus and room size?

By themselves, I would say they might struggle in such a big room. But if you're willing to add a sub or two, they might work. How high is the ceiling?

If you can afford it, I think M40.x will be a better fit for your room. But SHL5+ should work with a couple of subs.

P.S. I have owned SHL5+ for a few years now. They used to be in my main system (room: 20x15; 12 ft ceilings) and now in a much smaller home office (12x13 but 20 foot ceilings). They excelled in both rooms. Easily amongst the top 5 speakers I've owned thus far. Many speakers have come and gone but I could never bring myself to part with the SHL5+.

Mine are in a 24x20 room 4 feet out from the wall. I sit 10-12 feet away.  No issues. Plenty of bass for my liking. 

I have 8 ft ceilings and the room is dedicated to my audio. I was concerned about the bass. There not very big speakers but when I heard them in Texas they sounded so full and just right. 

With 8 ft ceilings I think they will work just fine. When I had them in my media room (20x15) they sounded really good. Adding two subs (REL S/510) took their performance to the next level. If you end up buying them, I highly encourage you to add at least one sub, if not two, at some point. They will easily sound twice as big. Good luck!