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

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \vec{m}_i^{\nu} = \vec{m}_i^{I} \cos(\omega_i^{\nu})
    + (\vec{k}_i \times \vec{m}_i^{I}) \sin(\omega_i^{\nu})
    + (1.0-\cos(\omega_i^{\nu})) \vec{k}_i (\vec{k}_i\cdot
    \vec{m}_i^{I})
  , \nonumber
  \end{equation}
\end{varwidth}
\end{document}
