Default font in Blog

Please have a look at the font of the blog posts on http://www.oorjan.com/home/blog/

I would like to make the “default” font size in the Blog posts like the one shown there, but this is actually “heading-4”. The “paragraph” is too small.

Is there a way to make this as the default size in the “posts”? I am going to add another author to the blog, and don’t want him to worry about the format, etc…

Thanks.