You could do what Mountain Lion does - 7px standard scroll bar, which grows to 11px on hover. It actually has a 15px clickable area on hover when you include the 2px margins. 15px visible all the time is a bit of overkill.

But don't make the mistake of implementing every little feature request or trying to fix everyone's problems (no offence to the op here). That's how bloat happens. Let uservoice do the talking to find out what the majority of people want, and focus on that.

Well, I don't really think 15px would look terribly wide, so rather than unnecessary complexity by expanding on hover, might as well always have it at that width. And you are absolutely correct that we cannot implement every single feature request. We'll most definitely be letting our users decide on uservoice.

15px would likely make it more usable on tablets. As an aside I wasn't able to resize a rectangle on an iPad 2. Good start!

Ah yeah, jquery-resizable is a tad difficult to use on touch devices. However, we are not planning to optimize for mobile devices anytime soon, since this is a tool people are mostly going to be using from their desktop.