How to determine smallest height of vertical scrollbar?

Other th开发者_如何学JAVAen creating a dummy NSScroller and incrementally decreasing it's height until usableParts returns 0, is there any easy way to determine what the minimum height of a vertical scrollbar should be so that usableParts will not return NSNoScrollerParts?

What I need to be able to do is limit the height of my window based, in part, on whether or not a vertical scrollbar will have usable parts.