When I set the minimum size of an ellipse in a tikzpicture environment, it expands into a circle if the inner content does not grow as large as the ellipse shape itself.
Is there any way around it?
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\usetikzlibrary{arrows}
\usetikzlibrary{shapes}
\tikzstyle{every node}=[draw, ellipse, minimum size=100pt,
align=center]
\node (a) {Alpha};
\node[left=150pt, below=30pt] (b) at (a) {Beta};
\end{tikzpicture}
\end{document}
It seems to me that, without something as wide as the ellipse length, TikZ is not able to define the width and height of the ellipse, but how can I set these variables?

minimum sizestyle simply setsminimum widthandminimum height(to the same provided value). If the contents are neither higher nor wider than those values the shape will be a circle. – Qrrbrbirlbel Mar 21 '13 at 02:54positioninglibrary. – Qrrbrbirlbel Mar 21 '13 at 03:41