I need to take a 1920 x 1080 project and make it 1920 x 540. The player it is being played back in will stretch it back to the 1080 size. Is there also something (like a setting) i should use to handle any distortion (anamorphic?). I'm totally new to this and it's kind of been dropped on me as a last second surprise! Any help would be much appreciated.
First question. Are you sure? That's very odd non standard behavior.
If it is for sure correct you need to create the 1920 X 540 composition, drop your original comp inside it (it's called nesting) and then Fit the nested comp to the existing comp squeezing it vertically. You'll find the automatic function under layer>transform or you can use the keyboard shortcut Ctrl/Alt + Cmnd/Ctrl + F to do it.
Make sure you understand exactly the compressor settings that should be used. As I said before, this is a very odd output size request.