\documentclass{article}
\newcommand{\mybox}[1]{%
\ifhmode
\unskip
\hspace{0pt plus 1 fill}%
\else
\noindent
\fi
\null\nobreak \hspace{0pt plus 1 fil}%
\fbox{\begin{minipage}{35mm}#1\end{minipage}}%
\ignorespaces}
%%% \parfillskip の値はデフォルト値(0pt plus 1 fil)であるものと仮定
\begin{document}
\mybox{1. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{2. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{3. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{4. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{5. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{6. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{7. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{8. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{9. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\mybox{10. Aaa bbb ccc ddd eee fff ggg hhh iii jjj kkk lll mmm.}
\end{document}
次のサンプルのようなことをお考えですか?