After checking the documentation, under vertex_size there is a note about the inner sep parameter. It seems that this parameter cannot be accessed through sage, however you may be able to tweak it in the output by adding the following line to the LaTeX output:
\tikzset{VertexStyle/.style = {inner sep  = 1pt}}

Right after the \begin{tikzpicture} would be a good place for this line. As a workaround, maybe searching and replacing the on the LaTeX string would be a way to "inject" this line to the output.
Hope this helps, otherwise please send some sample input to _latex_term for further assistance.