How to set the LaTeX font size in inkscape?
You can simply add a latex specification for the font size.
For example, you can use \fontsize in the "latex formula" box:
\fontsize{9pt}{1em} $\sum_{ij} w_{ij}$
Personally, I haven't used it yet, but the extension textext allows you to set an scale factor which:
(...) affects how much a newly created LaTeX object is magnified. You can later change/reset this via
Object->Transform->Matrix.