Hello Namita,
If the panel header font size could not be changed with CSS also, then i suggest that you create a 'Panel control look alike' build up, by making use of a simple Text View, a horizontal divider, and then add your items to it, which will make it look like the Panel control.. We did the same in one of our webpages. Also, the default color of horizontal divider can be changed to blue by CSS.
On a sidenote, please post your UI5 related queries to the UI5 space of SCN, you will get faster replies from people closer to UI5 development.
http://scn.sap.com/community/developer-center/front-end
Regards,
Rutika Bodas