You can change px to pt, but this will also affect any other instances of HTMLEditor you have added to your site (other than those included in WebAssist Solutions which have been uploaded to their own directory) as it is a common file.
In the block of code at line 54 of webassist/ckeditor/ckeditor.js, find:
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';
and change each instance of 'px' to 'pt'.
If you are using HTMLEditor as part of an emailing solution, there may be font-sizes pre-set in the template you are using (if you are).