Weight for a particular decoy depends on several things;
Line length/water depth, windspeed, current speed (if any), bottom type, and type of anchor.
I use custom-poured 4 ounce mushroom-type weights on lines of a length that are at least twice what water depth is, over very soft bottoms. They hold in 4mph currents (line length there is 4-5 times water depth) and in 30 mph winds.
Long lines, small decoys, soft bottoms, calm winds and mushroom or grapple type weights should allow 1 ounce weights to work fine.
Often wrong, never in doubt...