I agree on the two chickens being too small an amount. When we first bought ours, we only bought two. Even at a week old, there were problems with the two. After a few days, I added a third one and it immediately became one big, happy family. Three or more is definitely the way to go.
Now, if you are absolutely certain, without any doubt you will only be keeping a couple, I would go with a small coop. I'll base it off of three girls, since that is the minimum ideal. That puts your ideal coop space between 9 and 15 square feet. Personally, I found a 3'x5' coop is a really great space for three. During the days where they have to stay inside, it gives them a bit more room to move around and prevents pecking problems, but it's still small enough to tuck in to a nice corner. Also, I would put the roof at about 3' tall. They like to roost and the higher, the better.
Personally, I don't like the idea of my girls walking on chicken wire. Especially when they are smaller, there is a larger likelihood that their feet may be injured. It depends a lot on the quality and size of the chicken wire you buy, so be cautious. Take in to consideration your weather, also. Having several inches of bedding will provide warmth for them to snuggle in to during the cold and rough weather. It also helps eliminate drafts that may otherwise come through the flooring. You want them to have a draft-free area to be. And really, bedding can be completely free or inexpensive. The easiest form of bedding is dried leaves from your trees. Personally, I use a combination of dried leaves, pine shavings, wood chips from my woodworking, straw or hay from the nesting boxes, and the TerrAmigo whenever we've had chicks come through the house. With the exception of the TerrAmigo at about $10 for a 25-pound bag, everything else was free. The "natural" bedding will be a better insulator for them, too.
Before you get two involved, though, I have to warn you about chicken math. Absolutely everyone I know has bought a couple to start. And then a few months later, there is another breed or some other "cutie" at the feed store that you just have to get. I have yet to find anyone that only has three or four chicks. In my case, it was my wife that did me in. She was adamantly opposed to me having any, to the point of threatening divorce if I brought any home. After she saw them for the first time as chicks, she immediately fell in love and wanted to bring them home. She has been directly responsible for 9 of the 13 that we've bought. I was completely ok with what we had.
So the moral is to always make your coop twice the size you have planned. My 3'x5' coop is currently on it's way to being a brooder and I'm building a 6'x6'x52" coop in it's place.
