Or you can go with an even simpler option that let's you adjust CSS directly in the SimpleXML.
Add the following to your dashboard just before the closing </form> or </dashboard>. You can adjust the font-size value to suit your needs.
<row depends="$hidden_token$">
<panel>
<html>
<style>
.dashboard-row .dashboard-panel h2.panel-title {
font-size: 20px !important;
}
</style>
</html>
</panel>
</row>
