Bubble chart with wrong bubble scale


I am trying to create a bubble chart, I want to use the size of the bubble as a third dimension but I can not find a way to scale the bubble in the same scale as the axes! If I set the size vector, the bubbles are sized in pixels. What I would like to do is to set, for example, size=3 for a given bubble and have its diameter 3 units long with respect to the axis.