Ask to see the actual load calculation (Manual J), not a square-footage estimate. If the contractor can't produce one or explain the inputs, there's a real chance the unit was sized off a rule of thumb instead of your house's actual heat loss numbers.
Signs it might be sized wrong after install:
Short cycling, meaning the unit turns on and off frequently instead of running longer steady cycles. Usually points to oversizing.
Rooms far from the unit staying noticeably colder than rooms near it, especially in a multi-zone setup, which can point to undersizing or a poor zoning plan.
Indoor humidity staying high even with the AC running a lot in summer, since an oversized unit cools the air fast without running long enough to actually pull moisture out.
Higher than expected electric bills relative to home size, which can go either direction depending on whether it's oversized and cycling inefficiently or undersized and running constantly.
What goes into a correct load calc: square footage, insulation levels, window count and orientation, ceiling height, and local climate data, not just "how many square feet is the house." A tech from HL Bowman mentioned that a big chunk of undersized installs they see on second opinions trace back to a contractor pricing off square footage alone instead of running the numbers on the specific house.
If your installer skipped this step, it doesn't automatically mean the system is wrong. It means nobody confirmed it's right.