Larger will not hurt anything.

Seek local professional advice.

If the equipment is NOT nearby, is the air movement noisy? That's a sign of inadequate flow through the grate.

Ultimately the duct size may be a greater factor, along with leakage in the system, but if it's noisy and you want more airflow status quo, replace the louvered grid with a style called called "egg crate". It allows more air to pass compared to louvered ones which obstruct part of the opening. Just by looking at it you can see how much of the opening is covered.

I always agree it's better to get professional advice but unless you have additional, more substantial work to do or a regular relationship with an HVAC service, I think you'll have trouble finding a good contractor who would be interested in doing anything more than a Kentucky windage assessment. That's if you could even get one to come out. If you think the problem is enough that you'd be willing to spring for duct repairs and retrofits, different story.

I had all the attic ductwork redone by a pro. However he was not worried/interested in the return. I recently realized that the flex u Der the house is broken.
I’ll fix it myself and change the grid to a larger one.
Thanks!

16 x 16 is a total area of 256 sq/in. There are various rules of thumb and estimates, they tend to cluster in the 100-144 sq/in per ton. There is also the 2 CFM per sq/in theory, and the general tenet that a three ton unit would need around 1200 CFM, or 600 sq/in. So we have estimates calling for between 300 and 600 sq/in of return, and you have an vent sized at 256 BEFORE reductions for the edges and slats. I’d call it well undersized.

i would note that a 15” (15’ would be impressive) duct is a smudge under 177 sq/in, but the numbers are different for the ducts versus the intakes. Using an industry chart, a 15” round return duct is the absolutely smallest for 1200 cfm, and 1300 calls for a 16” duct. There is also a reduction factor in flexible ducts versus hard ducts.

How are you going to go larger? The opening that grill fits in is not adjustable without a sawzall.

Is the intention to cut a bigger hole in the wall and ducting?