Limit of font sizes

You may want to look at the CKEditor API, it has a section dedicated to your area of interest..
http://docs.cksource.com/ckeditor_api/symbols/CKEDITOR.config.html#.fontSize_sizes

The default setting for the font sizes is:

config.fontSize_sizes = '8/8px;9/9px;10/10px;11/11px;12/12px;14/14px;16/16px;18/18px;20/20px;22/22px;24/24px;26/26px;28/28px;36/36px;48/48px;72/72px';

You can modify the above line to fit your needs, then add it to your config.js
For example, if you want only sizes 8-16, your line would look like:

config.fontSize_sizes = '8/8px;9/9px;10/10px;11/11px;12/12px;14/14px;16/16px';