Checkout_my_Six
Touchdown! Greaser!
I would do (2) 9K units in the bedrooms and (1) 18 K for the larger open area. They will run/cycle as required. If they are too small they'll run constantly. You could always turn a unit off and leave a door open....but if they're too small you're stuck. I have an 18K in my wife's she shed 10x24 (it's well insulated). It's a little big and I might swap it out for a 9K but it doesn't short cycle....like I'd expect.
Also, make sure you're looking at least 23 SEER so that they provide heat at -5F.
Also, make sure you're looking at least 23 SEER so that they provide heat at -5F.