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

\begin{document}
\begin{varwidth}{50in}
  \begin{equation}
    \frac{d \vec{s}_{i}}{dt} = \frac{1}{\left(1+\lambda^2 \right)} \left( \left(
    \vec{\omega}_{i} +\vec{\eta} \right) \times \vec{s}_{i} + \lambda\, \vec{s}_{i}
  \times\left( \vec{\omega}_{i} \times\vec{s}_{i} \right) \right), \nonumber
  \end{equation}
\end{varwidth}
\end{document}
