I would build it as large as possible. Make sure it has irregular banks (not a big circle or square) gradual sloping banks and alot of water 1-3 feet deep, unless you are trying to attract divers. You can plant some crops around it, but I've had better luck with wetland and aquatic plants attracting them in. Keep in mind that ducks will land where they want to, your pond may or may not attract them enough to come in.
You will have to train your ducks to stay at your pond. Be prepared to have 50-75% mortality on ducks left to fend for themselves.
I'll start treating my wife as good as my dog when she starts fetching ducks.