"then the ac is too small..." Do you mean the air handler or the condensing unit. I have a 1400 under air sized home. The condensing unit is a Goodman GSC13 and the air handler is Goodman ARUF14/ARPT14.
The house cools at night and I can feel cool air coming out of the vents during the day, but the system continues to run...
The whole system must match in capacity- you cannot replace just one component here if this is an undersized system.
how do you determine the correct size? Please answer in lay terms.
A proper heat load calculation needs to be done by an a/c professional to get the right size system. An average size needed is one ton (12,000 btu) per 500 sq. ft. of cooled space.
The type of construction, exposure, insulation, etc all affect how much heat gets into your house and unless it's calculated properly then you are throwing money away on exorbitant utility bills.
They are both 3 ton units, and as previously stated 1400 sqft under air. Is there anything else that can be looked at? Thermostats?