Is there any rule of thumb about the size of a a/c return duct? I cannot get my house below 78 degrees when it is 90+ outside. Even when the house gets to 75 the thermostat will not turn off the unit. I have to dial in a higher number (78).

Thks

cobia13

((I cannot get my house below 78 degrees when it is 90+))

First check the air filter to make sure it is clean.

((even when the house gets to 75 the thermostat will not turn off the unit. I have to dial in a higher number (78).))

Sounds like your thermostat is out of calibration.

((is there any rule of thumb about the size of a a/c return duct?))

Yes there is but it is definitely not the way to correctly calculate the proper size.