The size of a Beamer slide is 128mm by 96mm. These dimensions are fixed and should not be changed.
It is permissible, however, to change the widths of the left and right margins. These are set to 1cm by default.
In the example below, we change the widths of the left and right margins to 6mm and 2mm, respectively.
\setbeamersize{text margin left=6mm}
\setbeamersize{text margin right=2mm}
It is possible to combine the two commands into one:
\setbeamersize{text margin left=6mm, text margin right=2mm}