Hi Praveen,
So when you say in point 1, that "it didnt work" what does it mean? What all changes did you make and where.
If you just increase the font size in your main CSS file (Mobile.css) within your internet service, it will get applied to all your templates. Just check the standard css classes used for your tags such as the body tag and so on in your generated HTML templates, and then you can add the following into your CSS file:
font-family: Verdana, Arial, sans-serif;
font-size:16px;
for those classes.
Remember to publish the css file as well as the whole internet service again.
Also, one you make this change, it will reflect on all the devices that the template runs on.