Definitely build bigger than you think you'll need. While I agree with several of the posters that chickens adapt to just about any size space, chicken math WILL catch up to you eventually. I thought I'd order only 3 to start out with in December. Ended up with 7 because I couldn't decide which ones I liked more and was absolutely positive as a first time chicken owner that I would end up killing half the chicks before they were a week old! I ended up killing ZERO! (Not complaining.) Then found 3 beautiful rare copper marans on craigslist yesterday, too. Yikes! See how the Chicken Math goes? 3 turned into 10 pretty easily. I ordered 2 more back in January for April pick up, which I now may cancel, but have 7 banties ordered for June. (Definitely see a bantie coop in my spring build!)
So, with that being said, I guess the REAL advice as to something I wish I knew when I was starting out is that ...
Chickens are addictive. Crazy addictive. Seriously. Really. Seriously addictive.