Swept mesh density dependent of a spatial function

Dear all,

I have a simple question: how can I define in swept mesh a function (spatial) of refinement? Just for example, I have a strip and I would have a very fine mesh only in the middle.
I have tried in this way: I set a parabolic function in Definitions>Analytic, 0.8+x^2 (supposing x is the direction of the swept), and then I inserted the function in Swept>Size>Max Element Size, hence an1(x). In this way I should get element of max 0.8 in the middle of the strip, and bigger going towards the sides.

But it seems that Comsol does not accept functions in element size parameters. What I am doing wrong?
Thank you very much

Fabio