1.Change the font size of your browser. In Internet Explorer go to File Menu - View - Text Size - medium( or as per your requirement)
In Mozilla Firefox go to Tools - Option - Content - Default Font - modify as per requirement.
2.If the above doesnt work and you have different font sizes on different pages of FB then clear your cache and cookies and temporary internet files from Tools - Options
3. If the above still doesnt work then upgrading to the most recent version of your browser would be advisable. Latest versions are available at :http://www.mozilla.com/en-US/firefox/personal.html
( for mozilla)http://www.microsoft.com/windows/internet-explorer/default.aspx
(for internet explorer 8)
Hope this helps