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.