1) How many square feet do ducks need when they are full grown?
3 sq ft in a 'house' 10sq ft in a run much the same as a large fowl chicken. I prefer to let ducks free range atleast a few hours though, but I know many people on here have house ducks 
 
2)Do they have a special find of feed for ducks? Like Cetawin said - Layena is awesome I feed it to my chickens and ducks and save on seperate bags of feed. But definately you'll have to add oyster shells for your chickens with it. Ducks don't need the amount of calcium chickens do, they need more protein and niacin than chickens do, but chickens can healthily eat their food. 
 
3) Do they need a duck house and if so what needs to be in it?  Similar to a chicken coop, but you'll waste space, material, and money if you make it tall as they stay on the ground all the time.  I use a large dog house/dog box its just a slanted roofed home made dog house thats really wide.  Mine free range 24/7 and get in it only when its cool at night or storming, otherwise they choose the open shed at my neighbor's under his tractor - lol. 
4) How long does it take a duck to mature? depends on some breeds but pretty much exactly like chickens - laying eggs @ 6 months.
5) Pros and Cons of ducks?  Pros : EGGS! Their eggs are awesome, larger than the largest chicken breed usually, tasty and rich, best for baking! Ducks are fun have alot of character and are just plain silly. In my experience I have had more tame ducks than chickens and ducks will follow you like puppies.  Their poop is watery as it is primarily water content and so it dissolves and disappears faster than chicken poo...
Duck pond water (kiddie pools/dishes they use and poo in 
  ) makes for GREAT and FREE fertilizer and recycled water for your plants if you garden!
Cons - VERY messy. They LOVE water and splash or spit it EVERYWHERE. You will need to look up duck waterers that reduce spillage and splashing if you aren't looking to clean out a brooder full of ducklings 2-4 times a day. 
6)  I currently have chickens, Do ducks need a completely seperate pen from the chickens? I don't suggest keeping them PENNED together - but free ranging or in an exceptionally large run/pen it can be okay - but again ducks make HUGE messes, much more than a chicken and I dont suggest them having the same housing as the chickens will be up in roosts and the ducks on the ground and so when the chickens poo...the ducks get pooed on. 
7) I would like a smaller breed duck, What kind of duck do you recommend?  
I have Magpies - a light weight duck and they are awesome! 
Khaki Campbells are supposed to be a really good duck too. 
Runners - I'm getting some from a fellow BYC'er next week and I am excited. I have heard they make one of the best pets.
Call ducks are small - but I don't know anything about them pet wise.
My favorite breed is one of the larger ones - Muscovy - great brooders, very easy going, lazy, and funny. but they are a big and heavy breed! But check around on here and see all the awesomely cute pictures and stories about muscovies you will quickly fall in love with them if you give yourself a chance!