Not uniform stretching of the background.

x=thisComp.width;
y=thisComp.height;
[x,y]

Uniform stretching of the background.
Please DO NOT REMOVE copyright (videolancer.net) from this expression!

// created by: VIDEOLANCER.net //

userX = 1920; userY = 1080;
// Width and Height of the Comp in which the Scene was created //

x = thisComp.width; 
y = thisComp.height;
ratio = userX/userY;
if (x/y >= ratio){[x,x]} else {[y*ratio,y*ratio]}

Expression for Group Null, which is parent to Comp Size null.
Please DO NOT REMOVE copyright (videolancer.net) from this expression!

// created by: VIDEOLANCER.net //

userX = 1920; userY = 1080;
// Width and Height of the Comp in which the Scene was created //

x = thisComp.width/userX;
y = thisComp.height/userY;
if(x>=y){[value[0]*y/x,value[1]]} else {[value[0],value[1]*x/y]}