How to change the default font size in my content

Try using Firebug with Firefox, or use the developer tools in Chrome or Safari or IE to see what’s loading on your site and how to work with and change the CSS and HTML. You will be able to see the body font size and where to change it in the style sheet.

I tried that long before posting, but I kinda have troubles.
Can you please suggest me what to change?

I’m sorry but it is beyond the scope of these forums to deal with general CSS issues. Try a dedicated CSS resource such as https://www.css-discuss.org/ instead.