Duct you Later...

I generally go by a minimum of 144 sq.in. per ton of cooling.
*Note* Your return can never be too big but it can be too small.

Don't know but I have a 3-ton HP with two 20"x20" returns, a 2 ton HP upstairs with one 20"x30" return.

Um, that's a lot of return filter area...
John

" *Note* Your return can never be too big but it can be too small. "

Ughh.. If your return air is too big, it wont flow back to the unit properly and it can ice up the unit.
Plus your not pulling back the stale air into the system to cool it in the area you want to pull from. Your temp readings in that area will take way longer to cool or heat in the winter time.

I have never ever in my life heard that happening from the return air being to big. How can having too much air available for the return freeze up your unit??

144 sq in per ton is a minimum by code where I am located.I normally oversize to around 180 sq in per ton.You can not freeze up a system with to large of a return.Altough static pressure must be considered when sizing a return.Some systems have no return duct at all.

There is a lot to proper ductwork design.
Below is a link to some basic ductwork design info.
udarrell _ 2007

Here is a link that might be useful: udarrell.com