\documentclass[a4paper]{memoir}
\usepackage{varwidth,graphicx}
\newsavebox\dlf
\newcommand\test[1]{%
\sbox\dlf{\begin{varwidth}{0.5\maxdimen}#1\end{varwidth}}%
\ifdim\wd\dlf>10cm%
\typeout{resizing}%
\resizebox{10cm}{!}{\usebox\dlf}%
\else%
\usebox\dlf%
\fi%
}
\begin{document}
\noindent\rule{10cm}{1mm}
\noindent
\test{
sdsdf\\
sdfsadf asdfds f\\
asdfsaf}
\bigskip
\noindent
\test{
sdsdf\\
sdfasd adsf dsf dsaf ds fljasdfl ;kfjdsa;lf j;sadkj ;asldjasldj ;dsapsdasdfsadf asdfds f\\
asdfsaf}
\end{document}