[NTG-context] scaling the mp figure in ConTeXt

Dear all, I use MetaFun to create a figure and embed it using \processMPbuffer[]. for example, \startbuffer[test] draw fullcircle scaled 3cm; \stopbuffer \starttext \placefigure[][]{} \processMPbuffer[test] \stoptext But sometimes, I need to rescaling the mp figure to fit the page. Is there a way of rescaling the figure at the stage \processMPbuffer or \useMPgraphic? In the above example, I got a circle of radius 3cm. How to get a circle of radius 2cm? Thank you for reading. best regards, Dalyoung