What size should the return air grill be for a 3ton ac unit?

referring to several manufacturer's recomendations, (Metal Aire, Titus etc.)for low noise you should use a 25x20 or 24x20 for a low nc. Of course, I'm sure every ac mechanic has his own "rule of thumb" and most would probably say you could get by with a 20x20, true, but again it's all based on air velocity thru the grille which relates to the noise created. Bigger is better. Good idea to line your ra plenum with something like eggcreate foam use on bedding to help with noise. I prefer a 2" pleated filter (no fiberglas) installed in a return air filter grille for easy change outs. They give best performance (dust trapping) for the money.