is my cold air return opening too small???
While it is true that the furnace blower in the furnace is designed to blow 400cfm per ton AT THE FURNACE, a 2-3 cfm per sq." air flow at the supply or return grille is correct and depends on duct size, shape, length, turns,etc., as well as how restrictive the grilles are (smaller spaced louvers = more restriction).
Important note: It's not really the size of the grille opening, but the opening behind it. Example: low wall grille that is 24"x10" (= 480CFM), but behind the grille, the wall space is only 3.5" deep; therefore 24" x 3.5 = 54x2 = 108cfm and we haven't considered the 1 or 2 vertcle 2" wall studs. Air flow is King.