If you are using the Jetpack plugin, you can change the font size of the LaTeX expressions by specifying an "s" parameter after the LaTeX code.
Here is the template:
$latex \LaTeX&s=X$
Where X goes from -4 to 4 (0 is the default).
You can modify your LaTex code as follows to use a large font size.
$latex \text{Product of Powers: } a^m \cdot a^n = a^{m+n}&s=1$
Set 's' to a bigger value, such as 2, 3, or 4 if you need a bigger font size.