Today's plastic decoys are very effective and can fool about any duck or hunter at just a few feet away. You can't own too many decoys. More is always better. If you are just starting out, buy as many as you can afford, then add a dozen or two each year. Soon you will have not quite enough.
Bigger decoys can be more effective by showing up better to ducks in the air.
Many times, I have had ducks glide over standard size mallard decoys and land among giant Canada goose decoys. But since today's standard mallard decoys are as big or bigger than real ducks, you can make just as good a showing with plenty of standard-sized ones. The decoy business is extremely competitive. That is an excellent buy. There are basically two types of keels.