Just follow these steps:
1. Locate file /qa-plugin/wysiwyg-editor/ckeditor/contents.css if using v1.7 or /qa-plugin/wysiwyg-editor/contents.css if using v1.6.3
2. Around line 10 you'll see the font-size attribute
3. Change it this way:
font-size: 24px !important;
Of course use whatever number you want instead of 24.