You add an expression to the text layer by alt/option clicking Source Text. You point the expression to the scale value you want to use. Let's say you are doing the scaling horizontally, then you would point to the X scale value. If your arrow was vertical you would point to the Z scale value. Now you add a linear method to the scale value and if you want whole numbers you round the result. Say you wanted the numbers to go from 1000 to 10 so your linear expression would say something like v = linear(t, 0, 100, 10, 1000)
The expression for whole numbers would look like this:
t = thisComp.layer("Shape Layer 1").transform.scale;
n = linear(t, 0, 100, 10, 1000)
The expression gets more complicated if you need decimal places. This universal counter from Dan Ebberts is a good place to start: Universal Counter
You also can apply the text effect "Numbers" to a solid layer then connect the value attribute to the scale attribute of the arrow. It's a quick and dirty solution but may give the result you want without having to write the expression yourself.
Cool. I'll try that. Thanks for the replies.