There are a LOT of duck housing threads floating around the duck forum, if you use the search feature, you'll be able to find a bunch of great examples...
Your ducklings will, of course, need to start out in a brooder. I use a big Rubbermaid stock tank lined with old towels that I hose down and replace with dry towels a few times a day. The ducklings in this picture had just about outgrown the brooder - they only spent the nights in there and were outside in the grass during the day:
My duck pens are 4'x8' plywood boxes with hinged roofs - here's a view of the inside:
Ducks don't need anything elaborate. They're just going to get it all wet and mucky anyways! You'll need good ventilation:
And you need to make sure it's predator-proof - my German Shepherd has tried to get into this pen on his evening walk a couple of times now, with no success, and we've had raccoons try to get in - but, so far, we've had no losses to predators:
A 4'x8' pen should accommodate 8 ducks perfectly - I had 10 in one of mine over the winter!
All my pens open to fenced-in runs (with kiddie pools to swim in!) that the ducks have access to during the day. I hope that helps you! Enjoy your ducklings!