The theme file only has settings that are overridden. The default file (ext-all.css) contains all the settings. Look at this file and search for "font" - you will find many places where the size of the font is set. For example, the following setting controls the size of the font in form fields:
.x-form-field{margin:0;font:normal 12px tahoma,arial,helvetica,sans-serif;}
If you need to override this setting, you have to provide your own values with exactly the same name (.x-form-field, in this case) in your theme file.