[NTG-context] How to set the caption length?

When a figure is placed, but this figure size is much smaller than the textwidth, the caption length will adjust after the figure size. \placefigure [][fig:xxx] {xxxxxxxxxxxxxxxxxxxxxxxxxxxx} {\externalfigure[xxx]} when set the figure size to an arbitrary size, the caption length will also change. But what I expect is, i change the figure size, but the caption size is always the textwith, or i can set it arbitrarily, for example 90% of the text size. I know it should be very easy, but honestly, i do not know.