There is no written code because the sizing can vary dramatically depending on window location(s), orientation of house, how good insulation and windows are, etc. That's why experts recommend a J-load calculation done by an HVAC professional.

Is your system struggling with heating? Obviously, that has a lot to do with how cold it is outside unless you are on pure electric backup heat, in which case sizing is the main variable.

Manual J for heat gain/loss. Manual D for duct sizing.

The square footage rules of thumb are bogus.

Upgraded windows, insulation, and sealing are the cheapest ways to cut your heating and cooling costs.

Buy a short term license for HVAC-Calc and do your own heat gain/loss calculations.

Here is a link that might be useful: HVAC-Calc

theboneskes

of course the nation including your area, the South and Southeast have been experiencing below average winter temps for an extended period of time.

Heat pumps are normally sized for cooling. Depending on your home's building characteristics and insulation qualities, a 2 1/2 ton system may be undersized. How old is system? Is it original to home? What size backup heat strip?

There have been many anecdotal stories in the past week about heat pumps struggling and not being able to maintain normal inside temperature settings. I wouldn't judge your HP's performance by this extreme weather. Wait until outside temps moderate and then make some observations.

IMO
Good Luck!