Here, its explained for you, it even tells you what pixel aspects (not device) just width and height

https://help.adobe.com/en_US/AfterEff...dea-7f3aa.html

if you leave the Device settings alone and just put in the width and height (uncheck Maintain width height ratio button) you should be okay.

Im trying to find out a file size (not pixel) so you can type in the setting without getting distortion, Ryan Church or Dylan Cole have a size for matte painting which is like a standard to fit a wide frame. I did write it down ages ago but cant find it, I will get back to you

Jay