I don't know about the ground space, but I would say it's probably smart to go bigger. I used to have a kiddie pool and I hated it. I had to dump it almost twice a day and only so many ducks could fit at once and it drove them nuts. I also found that when my drake tried to breed my hens, they were pushed under the water. It's actually seems safer for them to breed in deeper water.
I have a 220 gallon pre-form and I love it. I'm still tweaking the filtration system, but you'll always have to pump it out occasionally. At this point I'm doing it once a week until I have my pump back up and running, but when my bio-filter is going I can actually let it go almost 3 weeks or so. It still gets green, which is why I'm currently adding a bog filter as well to eat up nutrients. And now with my big pond all of my ducks fit at once and they can even dive under if they choose.