I'm currently planning a 4'x8' or 5'x8' coop on wheels, which I'm thnking is close to the smallest size practical.
You could put nesting boxes on one end and two roosting bars on the other. That would pretty much fill the space if you are doing things conventionally
For the nesting boxes, to have outside access you only need to make openings big enough for the chickens to go through (if exterior) or just big enough for you to see and reach inside, which would be even smaller (if interior, which I tend to favor). If the walls are alluminum sheet, then, Yes, you would want to add some kind of wood framing around those openings to avoid that sharp metal edge. The openings don't have to be rectangular, circular or rounded rectangle openings are actually better.
It would not be much work to get a sheet of plywood and some 2by3s for framing and add a floor, then you could put the whole thing on bricks or cement blocks (or on wheels, since the whole structure would not be very heavy.) If you go that way, though, you might want to think about some kind of ground anchoring, because of the structure being taller than it is wide.
You woud want to add some ventilation openings, and also make sure the doors are predator-proof. (No openings larger than even 1/2 inch.)
Here is a link to
a video about one guy's 4' x 8'coop , and how he gets
*lots* of birds in it by taking advantage of all that vertical space. I wouldn't go to that extreme but it's an example we can learn from.