Reduce size and justify font of caption using memoir

Question

I try to reduce and better align my caption text (and not the automatic label "figure.") text with memoir. I try this this without success, and i search in the memoir documentation, and try some commands, without success too.

My example code :

\documentclass[a4paper, 12pt,twoside, openright]{memoir}
\usepackage{polyglossia}
\setdefaultlanguage{french}
\usepackage{fontspec}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage[autostyle=true,french=guillemets,maxlevel=3]{csquotes}
\usepackage{xparse}
\usepackage{graphicx}
\usepackage[protrusion=true]{microtype}

\graphicspath{{FigureIntroduction/}}

\tolerance=1000

\sidecapmargin{outer}
\setsidecappos{t}

\makeatletter
%\renewcommand{\fnum@figure}[1]{\textbf{\figurename~\thefigure}}
\renewcommand{\fnum@figure}{\textbf{Figure~\thefigure~--}}
\makeatother

\captiondelim{\newline}
\captionstyle{\centerlastline}% for title
\setlength{\belowcaptionskip}{\onelineskip}
\setlength{\footnotesep}{\onelineskip}


\captionnamefont{\small}
\captiontitlefont{\small}



\begin{document}

\chapterstyle{bringhurst}

\begin{figure}[!h]
\begin{sidecaption}[fortoc]{Le "champignon informationnel" proposé par Frédéric Kaplan est révélateur de l'augmentation du champs d'expérimentation rendu possible par la numérisation des données, puis la simulation numérique.}[fig:I_Champi]
  %\resizebox*{\textwidth}{!}{\includegraphics{champignonKaplan.png}}
  \includegraphics[width=\linewidth]{champignonKaplan.png}
  \end{sidecaption}
\end{figure}

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some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...
some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...

\begin{figure}[!h]
\begin{sidecaption}[fortoc]{Le "champignon informationnel" proposé par Frédéric Kaplan est révélateur de l'augmentation du champs d'expérimentation rendu possible par la numérisation des donnéeeees, puis la simulation numérique.}[fig:Id_Champi]
  %\resizebox*{\textwidth}{!}{\includegraphics{champignonKaplan.png}}
  \includegraphics[width=\linewidth]{champignonKaplan.png}
  \end{sidecaption}
\end{figure}

some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...some text to compare ...

some text to compare ...

\end{document}

And the result :

Answer 1

The macro you are looking is \sidecapstyle (described in Section 10.10.1 Tweaks, page 199 of the memoir documentation) with default definition:

\newcommand*{\sidecapstyle}{%
%%%  \captionnamefont{\bfseries}%
  \ifscapmargleft
    \captionstyle{\raggedleft}%
  \else
    \captionstyle{\raggedright}%
  \fi}

You can introduce there your modifications:

\renewcommand*{\sidecapstyle}{%
  \ifscapmargleft
    \captionstyle{\justifying\scriptsize}%
  \else
    \captionstyle{\justifying\scriptsize}%
  \fi}

(\justifying requires the ragged2e package). A complete example:

\documentclass[a4paper, 12pt,twoside, openright]{memoir}
\usepackage{polyglossia}
\setdefaultlanguage{french}
\usepackage{fontspec}
\usepackage[usenames,dvipsnames]{xcolor}
\usepackage[autostyle=true,french=guillemets,maxlevel=3]{csquotes}
\usepackage{xparse}
\usepackage{graphicx}
\usepackage[protrusion=true]{microtype}
\usepackage{ragged2e}
\usepackage{lipsum}

\sidecapmargin{outer}
\setsidecappos{t}

\makeatletter
\renewcommand{\fnum@figure}{\textbf{Figure~\thefigure~--}}
\makeatother

\captiondelim{\newline}

\renewcommand*{\sidecapstyle}{%
  \ifscapmargleft
    \captionstyle{\justifying\scriptsize}%
  \else
    \captionstyle{\justifying\scriptsize}%
  \fi}

\begin{document}

\chapterstyle{bringhurst}

\begin{figure}[!ht]
\begin{sidecaption}[fortoc]{Le "champignon informationnel" proposé par Frédéric Kaplan est révélateur de l'augmentation du champs d'expérimentation rendu possible par la numérisation des données, puis la simulation numérique.}[fig:I_Champi]
\includegraphics[width=\linewidth,height=5cm]{example-image-a}
\end{sidecaption}
\end{figure}
\lipsum[4]

\end{document}

Notice, however, that now you might get (as the example code shows) bad spacing in some lines since the space reserved for captions is too narrow. Perhaps you should recondider the decision about the justification.