\documentclass[preview]{standalone}
\usepackage{varwidth}
\usepackage[utf8x]{inputenc}
\usepackage{amsmath, amssymb, graphics, setspace}

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    % \left| \left| \vec{F} \right| \right|_2 
    || \vec{F} ||_{2} 
    = \sqrt{\vec{F}_1+ \cdots + \vec{F}_N}
    \nonumber
  \end{equation}
\end{varwidth}
\end{document}
