Check my construction page for details and pictures, but in a nutshell,
what I found to be most effective are the punched aluminum soffett vents that you can buy at almost any hardware store or home center.
I used 4" diameter vents, one on each side of the wall, and glued them in place after using a hole saw to cut an appropriate hole. I put 2 vents down low, and two vents up high.
They aren't adequate by themselves, but they do provide constant air turnover
in my coop and will positively vent both moisture and gaseous ammonia, which are probably your two biggest concerns. In my climate, I am able to open at least one window daily, and in the summer it will stay open all the time so I do not have an accumulation of either;
but if I close up the coop on a cold night I still have air turnover without being drafty.
A definitive answer depends upon your actual design, but just about any kind of screened and/or stamped foundation vent or soffett vent works very nicely. If your siding is plywood it is easy to find suitable screws to attach it soundly.
On edit;
Again, it depends upon your individual coop design, and what actual materials you choose to use, but there is
nothing wrong with using commonly sold building materials, such as foundation and soffett vents as long as you use them properly. In my coop, when I close it up at night and shut the door, it is closed tight, and those vents make sure there is fresh air at all times.
As I stated,
these are not adequate by and of themselves, and they simply ensure that there is a positive turnover of air inside the coop at all times. By installing two vents down low, and another two vents of the same size up high, the heat in the coop that the chickens naturally release heats the air and it rises, and escaping out of the top vents while drawing fresh air in through the bottom vents. In this manner the air inside the coop is always refreshed, no matter how slightly, If necessary I suppose I can produce air flow anc air volume charts based on degree of temperature change, but I have satisfied myself that this system works and works well before I ever put my young chooks in it. I watch this system working every day.
I did a simple smoke test on this very installation with a couple of burning punks left over from the 4th of July, and with everything buttoned up tight and a 100 watt lightbulb in the brooder hood insidde, the lower vents were both sucking smoke and air in at the bottom and blowing it back out both top vents; using nothing other than natural convection currents inside the coop.
Chickens produce a lot of heat, and heated air rises. If you let in cooler air in underneath it, the air will slowly turn over while the coop is otherwise closed up tightly. Ammonia boils off at a low temperature so it is important to give it a way out of the coop.
Positive and constant air flow will accomplish that, and using these types of soffett vents work very will for me, and in my coop. I installed them carefully and securely. My chooks are in that coop right now, they are secure and the air inside is clean. It smells like chickens, and not chicken sewage, and that is the goal.