Question on Marketpress plugin

How can I change the font size in the Marketpress sidebar widget? Not the standard text widget, but the Global Product Category List widget. I want to change the size of the heading text, not the products listed by the widget.

For example:

I made the heading "Browse" and then it shows "Glassware, Home Decor, Toys, Men, Women, etc" i.e., it shows the categories.

The heading is a little to big for my tastes and I'd like to make it either an <h4> or an <h5>.

Thanks so much for your help!