\documentclass[12pt]{article}
\makeatletter
%% copié-collé du code de \fbox que je renomme en \MA@fbox
%% j'y change la dernière ligne \@frameb@x\relax} en \MA@frameb@x \relax}
\long\def\MA@fbox#1{%
   \leavevmode
   \setbox\@tempboxa\hbox{%
     \color@begingroup \kern\fboxsep{#1}\kern\fboxsep
     \color@endgroup}%
   \MA@frameb@x\relax}
\def\MA@frameb@x#1{%
   \@tempdima\fboxrule \advance\@tempdima\fboxsep
   \advance\@tempdima\dp\@tempboxa
   \hbox{%
     \lower\@tempdima\hbox{%
       \vbox{%
         \hrule\@height\fboxrule
         \hbox{%
           %% ici \vrule\@width\fboxrule
           #1%
           \vbox{%
             \vskip\fboxsep \box\@tempboxa
             \vskip\fboxsep}%
           #1%
           %% ici \vrule\@width\fboxrule
           }%
         \hrule\@height\fboxrule}%
                           }%
         }%
}
\newenvironment{changemargin}[2]{\begin{list}{}{%
\setlength{\topsep}{0pt}%
\setlength{\leftmargin}{0pt}%
\setlength{\rightmargin}{0pt}%
\setlength{\listparindent}{\parindent}%
\setlength{\itemindent}{\parindent}%
\setlength{\parsep}{0pt plus 1pt}%
\addtolength{\leftmargin}{#1}%
\addtolength{\rightmargin}{#2}%
}\item }{\end{list}}

%% maintenant on utilise \MA@fbox à la place de \fbox
\newcounter{UneLettre}
\newcommand{\UneLettre}[1]%
    {\stepcounter{UneLettre}\medskip
    \noindent%
    \bfseries\small{\MA@fbox{\arabic{UneLettre}}}
    #1\par}
\newlength{\letterwidth}
\newsavebox{\letterbox}
\newcommand{\OneLetter}[1]{%
  \stepcounter{UneLettre}\medskip
  \bfseries\small
  \savebox{\letterbox}{\MA@fbox{\arabic{UneLettre}}}
  \settowidth{\letterwidth}{\usebox\letterbox\space}
  \begin{changemargin}{\letterwidth}{0pt}
  \noindent\hspace*{-\letterwidth}\usebox\letterbox{} #1%\par    
  \end{changemargin}
}
\makeatother
\begin{document}



\OneLetter{balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla}

\UneLettre{balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla}

\UneLettre{balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla balbla
balbla balbla balbla}

\end{document}
