ZBrush Insert mesh scale

I need to cut a bunch of shapes that are as near to identical as possible out of a mesh. The problem I'm having is that the insert mesh scale seems to be tied to the position of the camera. I can use ctrl while drawing each insert meshe to keep a consistent scale, but as soon as I zoom out with the camera that scale changes.

Is there any way to lock that scale so it's always the same size, no matter where I move the camera? I can't see any obvious answers in the documentation.