How can I be sure my AC Unit is the right size?

If it is in design stage then You can be sure by making good load calculations and then re-check from some expert.There are some rule of thumb like tonnage and cfm per square meter or any unit of area.

If it meets all requirement then your unit will ofcourse be correct size.

If it is already installed then the best way is to check the performance during peak load if the performance is not good at peak load then ofcourse this is under sized.

twelve to fifteen sq mtr of area requires one ton and one ton is equivalent for  four hundred cfm for packaged or dx unit and  three hundred cfm for chilled network operated units.