\newlength{\pplength}
\newsavebox{\mybox}
\sbox{\mybox}{
    a
}
\settowidth{\pplength}{\usebox{\mybox}}
\the\pplength


\sbox{\mybox}{
    a

    a

    a

    a

    a
}
\settowidth{\pplength}{\usebox{\mybox}}
\the\pplength