Press "Ctrl-F" to open the Find tool and then enter "font-family" (without the quotations) into the field next to Find. Click "OK" to search for the text in the HTML code.

In the Web Content section, click the Font Size drop-down menu. You can choose between multiple sizes of text ranging from Very Small to Very Large, and the changes will take effect immediately.

Fortunately, Tumblr enables you to change the font size for your blog's text by giving you direct access to the raw CSS or Cascading Style Sheet code of your theme.