I'm using the beamerposter package but I have a problem with one equation:
\begin{equation}
P_\tau = pm_0 + \sum_{x=1}^{X} mr_{\tau,Lx} \cdot pm_{Lx}
\end{equation}
which appears like that:

The problem is obviously the size of the \sum.
The package include:
\usepackage[orientation=portrait,size=a0,scale=1.4]{beamerposter}
Please tell me if you need more information.
Edit: This problem appears when using \RequirePackage{lmodern} but without it, my fonts are ugly.
\usepackage{exscale}or\usepackage{amsmath}(which is recommended anyway when math typesetting is involved). – egreg Aug 21 '13 at 15:40\RequirePackage{lmodern}– Maxime Aug 21 '13 at 16:33