1200sf 2 story house, 18k centrally ducted Mistubishi. Most would consider this quite undersized! But we got our home sealed and insulated beforehand. We do well, but start to droop when it gets below -5 (currently mid sixties in the house and -13 outside). Last winter we had a -15 to 0 stretch for 48 hrs and the house stayed in the 60s.

Huge caveat here. We have an internal electric resistance duct heater that is NOT working and has never worked due to a broken relay we discovered today. It's 5 kW on a dedicated 30 Amp breaker. If this was working I'm sure we'd be up to 70F indoors no problem. Get your installer to test the backup electric heat!