Sub Articulo 138.1

I don't know why your trial with the caption* package went awry, but
the direct answer is:
\long\def\@makecaption#1#2{%
  \vskip\abovecaptionskip
  \begingroup \small %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
  \sbox\@tempboxa{#1: #2}%
  \ifdim \wd\@tempboxa >\hsize
    #1: #2\par
  \else
    \global \@minipagefalse
    \hb@xt@\hsize{\hfil\box\@tempboxa\hfil}%
  \fi
  \vskip\belowcaptionskip
  \endgroup %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
}
You could instead define \fnum@figure, as in:
\def\fnum@figure{\small \figurename~\thefigure}
This relies on the grouping provided by the figure environment.
It actually makes a bit more sense to redefine the float environment
so it declares \small instead of \normalsize, but that is defined in
latex.ltx rather than in the document class.