A 15' x 22' coop for 15 birds is a huge area. It's not like you are trying to squeeze 4 chickens in a tiny 4'x4' coop. That area under the overhang will never be blocked by snow. I don't think that has a ridge vent at the peak but can't tell. For winter I think you are good to go as is. You are probably good for summer too. A gable vent up at the peak of the front or back (or both) won't hurt for summer, hot air rises and that will get the heat out in summer. Use a louvered vent to keep rain out. If you do have a ridge vent you certainly don't need any gable vents.
You will need windows for light as you plan. You and the chickens need to be able to see in there with doors closed. Those windows don't have to be open in summer but a cool breeze on the chickens when they are on the roost might feel pretty good. In winter keep them closed as you do not want a cold breeze hitting them when they are on the roost.
The number of chickens does help. Due to the size of that building and the numbers of chickens I think you are in great shape.
You will need windows for light as you plan. You and the chickens need to be able to see in there with doors closed. Those windows don't have to be open in summer but a cool breeze on the chickens when they are on the roost might feel pretty good. In winter keep them closed as you do not want a cold breeze hitting them when they are on the roost.
The number of chickens does help. Due to the size of that building and the numbers of chickens I think you are in great shape.
