For things that only adjust the application (and don't affect content), subtlety is key.

Here's an idea for a knob solution (with a cursor change) to consider...



...and maybe an even smaller knob would do?

If you decide to keep it to just the thin line, maybe putting a few smaller dots in the middle of the line (to indicate that something can happen) might be nice. The cursor changing to an east/west is the bare minimum needed though.

I've never found a reason to adjust the width, to be honest. I like how it forces me to name things short and succinctly. I try to subscribe to the KISS method.

-- Cody