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

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \vec{k}_i =  
    \frac{\vec{m}_i^I \times \vec{m}_i^F}{\left|\vec{m}_i^I 
    \times \vec{m}_i^F\right|} 
    %&{\rm ~if~}& \vec{m}_i^I \times \vec{m}_i^F  
  , \nonumber
  \end{equation}
\end{varwidth}
\end{document}
