You have come to a great place to get lots of information.
https://www.backyardchickens.com/atype/2/Coops
This is the absolute best place to start looking at ideas.
For 13 you will of course have to take into account how much time they will spend in the coop. Where I am at it gets really cold in the winter and there are times when we have a lot of snow. I had to take that into consideration when we made our coop.
Some breeds do not like being confined and will need more space then others that are fine with limited quarters. What breeds did you choose?
When I had 15 large hens the 8x8 coop was to small for them. They had fights and showed signs of stress.
Prefab coops for 13 (advertised to hold that many) will actually comfortably hold about half that. For what they cost you would be better off getting a shed kit and modifying it.
http://www.homedepot.com/p/Handy-Ho...t-x-10-ft-Wood-Storage-Shed-18250-1/100350323
I built one exactly like this in the yard for storage. It does not come with the floor or shingles but was easy enough to do with all the precut parts. I think our total came to 1100 with the 3/4 ply floor and shingles to match the house. Adding windows would be easy as would adding ventilation for chicken housing. If you are reasonably handy and have a helper to handle the heavy parts it goes up fairly fast.
If you look around you may find one on clearance that was a floor model too. Those I would have delivered of course as they are assembled.
That is my best advice on a coop to hold that many chickens.
Wishing the best for you and yours,
Babs
