How to change default font size in tinymce?

You need to use the content_css setting of tinymce to set a custom css file of your own (make sure this setting points to a valid location of a css file). This file will be inserted in the editor iframes head after all other css settings(files from the core) are inserted there when initialising tinymce – thus all settings you place in your file will overwrite the settings made before (by tinymce).

Example: Setting the default font-size to 11px. Content of a custom css file (i named it content.css in my installation):

body {
    font-family: Verdana,Arial,Helvetica,sans-serif;
    font-size: 11px;
}

How to use this setting:

tinyMCE.init({
 ...
 // you do not need to include it yourself, tinymce will do this for you, 
 // but you will need to give tinymce the location of your css file
 content_css : "http://www.myserver.com/css/content.css", 
     ...
});