Change Sidebars-Widget tittles font size

I’ ve read many posts and y have inspected with the firebug the element tittle in the sidebar widget woo commerce or in the blog sidebar widget. The titles font sizes inside them are too big.
Finally i’m just totally unable to change them in theme options— CSS style options. Nor I find them in the style CSS. ¿It is possible to make the changes? Thanks a lot