Usually the decision point is a compressor on an older unit: a big repair on tired equipment, or a new system. You see both numbers, plus how old the unit is and what shape it's in.
A lot of installs just copy the size off the old box. We size from the house: square footage, duct condition, attic insulation, how much sun hits the west wall. Too big and the system short cycles and leaves the air sticky. Too small and it never shuts off. Arlington has whole streets of homes from the same builders and decades, and we've measured plenty of them.
Call us for a second opinion and a straightforward comparison.
Call (682) 206-0032