How can I change this Latex poster size to A3?

try to add this to your script (taken from here with relevant changes)

usepackage{pgfpages}
pgfpagesdeclarelayout{resize and center}
{defpgfpageoptionborder{0pt} }
{pgfpagesphysicalpageoptions{
  logical pages=1,%
  physical height=pgfpageoptionheight,%
  physical width=pgfpageoptionwidth%
 }
 pgfpageslogicalpageoptions{1}  {%
resized width=pgfphysicalwidth,%
resized height=pgfphysicalheight,%
border shrink=pgfpageoptionborder,%
center=pgfpoint{0.55pgfphysicalwidth}{0.45pgfphysicalheight}
} }
pgfpagesuselayout{resize and center}[a3paper,portrait]