\newlength{\captionwidth}
\setlength{\captionwidth}{.85\hsize}		% Whatever your heart desires
\makeatletter
\long\def\@makecaption#1#2{
\vskip 10pt\setbox\@tempboxa\hbox{\small \bf #1: #2}
\hbox to \hsize{\hfil
\ifdim\wd\@tempboxa>\captionwidth
\hbox to\captionwidth{\hsize=\captionwidth\vbox{\unhbox\@tempboxa\par}}\else
\hbox to\captionwidth{\hfil\box\@tempboxa\hfil}\fi\hfil}}
\makeatother