If you have the tinymce editor enabled for comments boxes, and you wish to increase the size of the text, then you will need to do the following.

To alter the size of the text in the comments area - if you have tinymce enabled for comments.

Edit /tinymce/themes/advanced/skins/default/content.css

look for the following element

body, td, pre {color:#000; font-family:Verdana, Arial, Helvetica, sans-serif; font-size:10    px; margin:8px;}

Simply change the value in red to the desired setting.