The filter size is determined by the amount of air flow which must pass through it. You have to determine when the air flow is the highest between the furnace and AC. The formula I use is for a filter rated for a velocity of 300FPM:

air flow (CFM) / 2.08 CFM/sq. in. = filter area (sq. inches)

Let's assume you have a 3 ton AC which would normally be using 1200CFM of air. The filter area calculation would be:

1200CFM /2.08 CFM/sq. in.= 577 sq. in.

A 20 X 24 filter (480 sq. in.) would be too small. You would need a 24 X 25 filter. The fact you added 500 sq. ft. of area does not change the filter requirements. If you are going to change the filter then see if you can fit a 4 inch media filter. They are less restrictive and last longer than the one inch.

Thanks Mike.

Would the CFM info be listed somewhere on the unit?

It is probably not listed on the unit. It may be in your furnace manual. An air flow of 350-400 CFM is what is required per ton of cooling. Usually the air flow for the air conditioning is higher unless you have an a very big furnace.

Thanks again Mike.

Chances are if they only installed one return on a 3 ton unit they also shortchanged the return ductwork. Adding a 2nd return will allow you to get the full 3 tons you paid for. Right now it is probably like most systems that only deliver 2/3's of their rated capacity.

FWIW my house is 100sqft bigger and cools with a 2 ton AC. It will keep it 75 indoors on a 100degree day.