1) you control this in the style sheet
2) look at one of your pages in a browser
3) view source and find the bit of code that surrounds the text content you want to control
4) what is the tag? (eg. ? or <h2>?
5) does the tag have something like <p class="post"> or <p class="entry">?
6) if so, look in your style sheet for .entry or .post
7) what is the font-size? em? px?
8) modify to suit
9) if you are editing via ftp, then upload the newly edited style sheet. If from the admin screens (templates), save the changes
10) view site and refresh the page. did it change?
It is NOT a good idea to just add more rules at the bottom of the page. Between cascade, inheritance and specificity, you could get wildly varied results, particularly if
a) the font size is set in em lengths and
b) the xhtml is writing <br/> instead of