I had my Home AC serviced today by a new company and one thing I was told is the current Plenum is to large.

The return Plenum is 48x24x20.

The Supply Plenum is 88x24x20. the first 44 inches have no attached ducts.

From what I have read the supply and return plenum should be balanced. This seems like a correct assessment but asking the texags experts.

This is a 5 ton unit.

I don't think they need to be the same size at all in fact, very rarely have I ever seen them the same size.

If you haven't noticed issues in the house I wouldn't worry about it in the slightest. The only thing I could think of is if certain rooms the air just trickles out of the diffuser.

The AC guy said that the Plenum is to short before the first branch per ACCA. Everything I can find indicates that the length should be 1 to 3 times the width. Not sure the minimum length to the first branch.

Then the concern would the the air being too turbulent for the correct amount to flow into the first branch. That rule of thumb doesn't really account for the main being oversized. Another common rule is straight duct 2-3 times the diameter of the tap. (I work with commercial systems though). Again if you haven't noticed any issues I wouldn't worry about it.