I think you just need a simple expression on the scaling, where you can enter a pixel value, something like:
pixelWidth = 500;
layerWidth = thisLayer.width;
ScaleWidth = 100*pixelWidth/layerWidth;
[ScaleWidth,ScaleWidth]
In this expression, 500 is how wide you want the layer to be. AE then gets the actual width, converts it to a percentage, and applies it to both the X and Y scales.
If you want it stretch the Y scale, then you just need to repeat the top three lines, with a different variable name and drop it into the coords as the second value:
pixelWidth = 500;
layerWidth = thisLayer.width;
ScaleWidth = 100*pixelWidth/layerWidth;
pixelHeight = 500;
layerHeight = thisLayer.height;
ScaleHeight = 100*pixelHeight/layerHeight;
[ScaleWidth,ScaleHeight]