How can i change the paper size in tikzposter?

The tikzposter class loads geometry, so you can use its syntax to specify different paper sizes:

\documentclass{tikzposter}
\geometry{paperwidth=4in,paperheight=3in}

\usepackage{lipsum}

\begin{document}
\lipsum[1]
\end{document}

10.2cm is 4in and 7.6cm is 3in

This is not enough. tikzposter uses \TP@visibletextwidth and \TP@visibletextheight to compute things like column widths etc., and it's computed from the default page size by the class. So, after you set the paper height and width you also have to say

\makeatletter
\setlength{\TP@visibletextwidth}{\textwidth-2\TP@innermargin}
\setlength{\TP@visibletextheight}{\textheight-2\TP@innermargin}
\makeatother