The code for scale would be:
rect=sourceRectAtTime(time, false);
rw = rect.width;
rh = rect.height;
finalWidth=60;
finalHeight=40;
[finalWidth/rw*100, finalHeight/rh*100]
You can replace the finalWidth and finalHeight settings with a slider if you want to be able to change them.
The scaling is not proportional with this expression, it will force the layers to be that size regardless of original dimensions. Layers will scale around their origin points.
You can put this expression into one layer scale property then copy and paste to all other layers.