set scale in pixels in After Effects using Expressions
The size in pixels is easily computed as the source's original dimensions multiplied by the scale. Or in other words 

p = d × s


where p=pixel dimensions required, d=source dimensions, s=scale.

So to set the scale so that it takes a given source and makes it conform to a desired dimensions:

s = p / d


In an expression on the scale property you'd use

var p = [123, 456]; / desired output dimension
[ p[0] / width, p[1] / height ] * 100; // the *100 is because scale is a percentage


To be really fancy you could repurpose the scale value input so that it controlled the pixel dimensions. This means if you want the layer's dimension to be [123,456] you'd just dial it in in the scale control:

var d = value; //returns the pre-expression value of the property
[ p[0] / width, p[1] / height ] * 100