While adding the beamerposter package, provide the required parameters.
\usepackage[orientation=landscape,size=a1]{beamerposter}
You can also customize the size of the poster.
\usepackage[orientation=portrait,size=custom,height=110,width=80,scale=1.4]{beamerposter}
The height and width dimensions here, are in cms. The scale
is used for the font size.