I'm planing on having 4 ducks, and 4 chickens and i don't know the smallest(or a good size) i can make the duck and chicken pen
Well are you talking coop or run when you say "pen"? Also I've heard ducks and chickens in close proximity together can cause worm problems. My chickens and ducks free range together and I've never had a worm epidemic; but someone I met told me their ducks and chickens were cooped up together and within a few months they were all dying from worms. Something to look into for sure.
Go with as much space as you can - the birds will be happier, and you will have the option to get more birds down the line. For the coop / inside shelter, you will want 6x6 or so. Target 4 sq ft per bird. For the outside run / pen, 8x10 will give you 10 sq ft per bird. If you go too small, the pen and coop will get dirtier faster, and you may end up with healt issues, or have birds picking on each other from crowding issues. I house 18 chickens and 8 mallard ducks together, and have had no issues. The chickens take the upper roosting bars, and the ducks nest underneath the nesting boxes (away from the roosting bars so they don't get pooped on from above! I also have 7 pekin ducks that have their own coop, but share the same outdoor pen as everyone else.
Make it as big as you can. If you haven't had ducks before, keep in mind they will make a MESS of their water. I like to give double the space for ducks as I would for a chicken. If you plan on giving them something big enough to bathe in, make sure you set things up so you have good drainage. Even when I only had 2 ducks on one kiddie pool, I had to completely dump it every 2-3 days which makes for a lot of water to go *somewhere* (And I've never had problems with parasites in my mixed flock, but mine also free-range all day & only get locked into the pen with attached coop at night which keeps things cleaner.)
