We can use KOMA-script to help us.
As explained in How can i change the fontsize with KOMA-script? we produce a new file.
\documentclass{article}
\usepackage{scrfontsizes}
\generatefontfile{theguy}{36pt}
\begin{document}
\end{document}
Running the above example will give us a file theguy36pt.clo. That file can be input into our document, but since it contains internal macros, we need to protect it using \makeatletter and \makeatother.
\documentclass{tikzposter}
\usepackage{lmodern}
\makeatletter
\input{theguy36pt.clo}
\makeatother
\usepackage{blindtext}
\begin{document}
\block{Walzing Wombat}{\blindtext}
\end{document}
There is quite a lot going on in the file, stuff that is not needed for a poster. You should of course take a look at this file and adjust it a bit. There is no real use for floatseps within a poster, but the gap between text and an equation still matters.
normalfontusing any arbitrary size, but have to redefine the relative font switches as well.\renewcommand{\normalsize}{\fontsize{24.88}{30}\selectfont}<- this is actually the default for 25pt. – Johannes_B May 07 '15 at 17:03