For my 5 girls we have a 4x8 sheet of plywood for example. Its plenty big enough, seemingly crowded when you get the all in there. Personally, I would reccoment 8x8 then you can have brood pens, feed bins and that type of thing for your needs, even if you don't use them as bird usage, storage of shavings and feed is something I wish I'd done!!!
Good luck remember to share pics!
The rule of thumb is 4 square feet per chicken and that is pretty safe, but that depends on what you are putting in there and how much time they are going to spend in there. More is always better. You can get by with less if all they ever do is roost in there. If they cannot go out to a run most of the time, it needs to be larger.
Since most building materials come in 8 feet lengths, I don't think I'd go smaller than a sheet of plywood (4'x8") for 5 chickens for a standard coop. I don't think you save that much in building materials. If I needed to get inside it to clean it, I want some elbow room. For a "playhouse" type coop, one of those that is elevated in a run with them spending a lot of time inthe run, I'd go smaller so I could reach in to clean it.
You did not ask for it, but I'll include a link to a thread about building a coop that could come in handy.
Or 4x5. Bigger is better just in case you decide to get more chickens. I have built custom coops that were 5x8 and the person who wanted them was putting 15 birds in each, (if you look at my pages here you can see them) but like others have said they had a large run. My personal coop is 3x5 and I have 4 birds in it and they have plenty of room.
You also need it big enough to easily service. Think about yourself out there summer and winter and in the rain. For those of us with a winter, 15 sq ft is more realistic per bird, for those times when it's savagely cold and they have to be in.
I would like to invite you to my BYC page showing some very practical inovations. The best of ideas was the slide-out bottom. I originally did this so they could walk on the 1/2" x 1/2" welded wire when it gets really hot, but the greatest benefit is the ease of cleaning.
Bigger is better, like closet space you never have enough. I have a 6x8' coop, which is fine for the 8 chickens that sleep there but I have 10 chicks in the brooder in the basement and an incubator full of eggs. What to do, what to do. I have several building projects to start on next week.