Loc: Southern NJ, USA
No simple rule of thumb. Outside temperatures vary, wall insulation varies, window area and type varies, how many walls, floors, ceilings have outside exposure and how many adjoin other heated space, what's the wind like, how much solar heat gain do you get, etc.
The good news is with electric heat, if you oversize it doesn't hurt anything or cost anymore to operate it just operates for shorter cycles.
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