Why square footage alone is not enough
Two 500-square-foot rooms can have very different loads. One might be shaded, well insulated, and surrounded by conditioned space. The other might have west-facing glass, a vaulted ceiling, an exterior door, and an attic above it. Their floor area is the same; their heat gain and heat loss are not.
A good preliminary tool asks for more than area. It should at least expose assumptions for ceiling height, insulation, sun exposure, occupancy, windows, and climate. Even then, it cannot see construction details the user did not enter.
Cooling load is only half the question
Heat pumps are reversible systems, so a project that depends on the mini split for winter heat needs a heating design check too. Nominal capacity is not necessarily the capacity available at the coldest expected outdoor temperature.
When winter heating is important, look for manufacturer expanded performance data or certified data that shows capacity and input at relevant outdoor temperatures. Cold-climate products may maintain more capacity as temperatures fall, but the exact model still has to be checked.
What to hand your contractor
Bring a room list, dimensions, window sizes, insulation information if known, occupancy patterns, desired indoor temperatures, and any unusual loads such as a server rack or large kitchen equipment. Ask for the design outdoor temperatures and the calculated room-by-room loads.
ENERGY STAR advises proper sizing and specifically points homeowners to Manual J. That is the right mental model: calculate the building requirement first, then choose equipment that can meet it.