Hi Dan,
Here is the expression –
// Scale with control layer
// Applied to scale parameter
// Point1 is current layer, point2 is control layer
point1=thisLayer.position;
point2=thisComp.layer("Drag Me").position;
// Find the vector between the 2 points
delta=sub(point1, point2);
// Now find the length
distance=length(delta);
//Remap distance to large scale when near, small when far
linear(distance, 0, 100, [250,250], [75,75]);
Regards
Mark