Larger duct sizes will deliver higher CFM. If your flex is poorly supported or not tight,or has kinks, this will severely restrict CFM. Consider using hard pipe/duct.

The answer to your question is a little more complicated.

V

Ignoring the flex, what is the max CFM that the in-wall duct would allow to be supplied through the return?

"Ignoring the flex, what is the max CFM that the in-wall duct would allow to be supplied through the return?"

Not a PRO, but I'll give you my take on it. The wall cavity (assuming 2 x 4s, is 3.5" deep, by about 14.5 wide. The nearest commercial size rectangular duct is 14 x 4. So let's assume they're about equal.

a 14 X 4 duct CAN carry 200 CFM. Notice the word 'CAN'. The actual airflow will be dependent on the static pressure in the duct system. So a return duct with turns, long length, and other restrictions (flex and filters, notably), will tend to have a higher SP, and prevent the duct from carrying its full potential of air.

Having a variable speed blower can help to overcome some duct problems by ramping up in an attempt to deliver the set airflow.

By enlarging the duct downstream of the wall, you are allowing the velocity of air to be decreased, which in turn, decreases the SP. So it can help, somewhat.

It would be beneficial to add additional return(s).

V