need to know how to put in a little chicken door and the size?

I don't understand exactly what your set-up will be, but I'll try a few of these.

I made my pop door about 12" x 12". It is plenty big enough, but they only have to jump up about a foot from the run to get back into the coop. Mine is hinged, not a slider. You can do it either inside or outside, but it might be better to mount an automatic one on the inside to keep the mechanism out of the rain if you have room. I would make it about a foot high off the coop floor. They can easily jump that high once they get a few weeks old. Since you don't know if you will put any bedding in there, I'd put it a bit high so they don't scratch the bedding out.

 

Decide on inside or outside track for your pop door based on what's convenient for you, since you will be closing/opening it (unless you're doing an auto door, and RR's recommendation made a lot of sense). Before we did our auto. door, we had an inside slider rigged up on a pulley system so that it could be opened or closed still from outside. That way we didn't have to enter the run or the coop to open/close their door. But we could have done that with an outside mount too...

My pop door is only 3 - 4 inches off the floor, and I wish I'd done six instead (because bedding does get into the bottom track sometimes). My pop door is about 13 inches tall by 12 inches wide. My breeds are in my signature...and I could have gotten by fine with a 10 inch width. If you're not putting bedding on your coop floor (and I'd STILL paint or put down vinyl flooring), make sure your roosts aren't too high, as it's hard on their feet to jump down onto solid wood from up high. Even if you place a chicken ladder going up to your roosts to help, not all of them will use it.