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.