[NTG-context] How does \setupfloat work?

I'm trying to set a default width for figures with \setupfloat. However, the command doesn't seem to be picking up the width specification, even though the manual indicates it accepts a width= argument: \setupfloat[figure][width=2cm] \starttext \placefigure{}{\externalfigure[picture.png]} \stoptext picture.png just gets inserted at its native size, rather than scaled to have the specified width. What am I doing wrong? (setting minwidth and/or maxwidth doesn't seem to work either) Thanks in advance for any assistance. Mark