We used 2x4 lumber for the bottom frame of our coop, and metal corner braces and screws to secure it together. 2x4s make a good size for the bottom of the coop.
Make sure the bottom frame is square. Use a square or the corner of a piece of plywood to check each corner, and measure the diagonals to make sure they are the same length.
If you'll be using your coop with a surrounding fence, figure on a minimum of 3 square feet per bird. This movable coop is 9 feet by 8 feet, so a good size for about two dozen chickens. Right now we have 34 chickens in it with no problems - it's summer and only at night are they all in it at the same time.