I'm trying to do a relatively simple green screen effect. The first one I tried worked beautifully, because I had a full screen green backdrop behind the subject for the shot. This one (pic) was different. I only had so much space and room With GIMP, I know there's a way to pick a backdrop color and have it fill the screen. Is there a way to do the same thing with PE11 or AE CS6?
Use the Garbage Mattes (in the Key effects) to remove the area outside the green screen.
Okay, that solves the sides, but I suspect I need to do something to make the green even so that the background will look right?
Well, that's not something you can really do in post-production. You'll need to make sure your backdrop is evenly lit and free of wrinkles when you shoot the movie, otherwise no amount of chroma keying is going to be able to fix it.