My pop door is too big, about 18 X 24 in. I am working on modifying it. Guys who built it didn't know about chickens, and I didn't specify exactly. Apparently they thought chickens are HUGE! LOL! But no problem, overall they did a good job!
I agree with making the "people door" (and both doors, actually) about 6 inches up from the floor. Keeps the shavings in better, and also helps with opening the people door. Mine swings IN, and it's hard to open it with 6 inches of shavings against it. I would recommend having the people door swing OUT. Mine doesn't because I used a recycled door from the house, frame and all, and it would have been too complicated (for me) to reverse the swing of the door. I've been told that it's easy, but........
Just had an addition built on, and this time had the door built in higher. It still swings in, but clears the shavings nicely. (I used the same door, but mounted it higher.)
That brings up another thought. Be sure you build big enough for all the chickens you have or might want to get in the future. Better too big than too small. You can always block off part of it for storage, but making it larger is difficult and expensive. Been there, done that.
Best of luck, and be sure to post pix when you are done.