Sizing HVAC: 3 vs 2.5 ton AC, 60k vs 40k BTU furnace for 1200sqft?
What was the previous size of the AC?  It's typically encoded in the model number, and there are online decoders for your manufacturer.
The size of your AC is based on the heat leakage for your home; but, many AC contractors believe that adding an extra 1/2 ton doesn't hurt.  It is generally a safe practice if it is only done once; but, if it's already been done, over cooling a home can have some pretty nasty consequences,  Humidity build up (like the water on the outside of a cold soda can) can collect within the home due to air temperature being dropped to rapidly.  This typically occurs behind the sheet rock, where it won't be immediately visible.  In addition, your unit will run for very short periods as it cycles, likely leading leading to more wear on the installed system.
In any case, the home hasn't moved, the insulation hasn't changed, and unless you had a bad system before, the new one shouldn't require a new tonnage rating unless it was mis-rated previously, or you intend to maintain the home at a lower temperature than before.
Should you believe the home was mis-rated, pay a company to have a &quot;Manual J Load Calculation&quot; done on the home.  If it is possible to have this done without installing an AC at the same time, you'll have a better confidence it isn't influenced by the impending sell.