I made my doors big (12"W x 20"H), which is great in summer because, when open, they let plenty of air circulate. In winter, though, they let in plenty of cold drafts, so I solved that problem by making an "insert" from scrap plywood that I screwed to the interior door frame. The opening on it is only 9" x 9", but even my big fat Australorp gets through it just fine. When I want to remove it I'll just unscrew it.
I also use a length of paracord strung thru some screw eyes to open and close the door from outside the run. It's nice not to have to walk through chicken poop to open/close the door. I also got some of those el- cheapo magnetic catches like you use on kitchen cabinets for the pop doors. They aren't REAL strong, but they do keep the wind from blowing the door open.
Except in cold weather I don't worry too much about closing the pop doors because my runs and coops are completely predator proof... that way the girls get up and go to bed on their own schedule.