tinymce/themes/advanced/skins/o2k7/content.css
add the following code to bottom
#tinymce { font-size: 15px;}} //change the variable according to your need
tinymce/themes/advanced/skins/o2k7/content.css
add the following code to bottom
#tinymce { font-size: 15px;}} //change the variable according to your need