sum symbol in tikzposter too small

It's a known bug of the Latin Modern fonts, which only supply the lmex font (extensible symbols) at a fixed size.

Note that this affects all “large symbols”, such as \sum, \int, \prod and so on, including extensible braces.

\documentclass[25pt, a0paper, portrait]{tikzposter}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage[T1]{fontenc} 
\usepackage{lmodern} 
\usepackage{amsmath}
\usepackage{amssymb}

% declare `cmex` to be arbitrary scalable
\DeclareFontShape{OMX}{cmex}{m}{n}{
  <-7.5> cmex7
  <7.5-8.5> cmex8
  <8.5-9.5> cmex9
  <9.5-> cmex10
}{}

\SetSymbolFont{largesymbols}{normal}{OMX}{cmex}{m}{n}
\SetSymbolFont{largesymbols}{bold}  {OMX}{cmex}{m}{n}

\title{lalala}

\begin{document}
\maketitle

\block{test}{
  \[
    \Psi = \sum^{n_{max}}_{n=1}C_n
  \]
}
\block{again}{
  \Huge
  \[
    \Psi = \sum^{n_{max}}_{n=1}C_n
  \]
}
\end{document}

Never use eqnarray.