More pertinently there is a conflict between your DTD and mime type.
In your theme folder you will find a file typically called style.css Depending on what part of your site you want to alter (the posts, the sidebar, the header, the footer, etc.) you will need to edit entries in this file to change the size of the text. Typically the main body of the site will be handled either under an entry called body or content. In this entry you’ll probably see a line with “font-size:” listed with either a number with the letters px after it or a number with the letters em after it. This number determines the size of the text. Making the number bigger makes the text bigger, and vice versa. You’ll need to play around with this to get the sizing the way you want it.
I’d suggest looking for a “CSS” tutorial to really understand how this works–there are tons and google will get you to one quickly.
