When creating a Nintex Form it may be desired that the default font size is set for a Multi Line of Text control that is configured for Rich Text. 
 

While selecting the Multi Lines of Text control in the Nintex Forms designer the ribbon will show attributes that can be set in regards to font, font size, colour etc.
Unfortunately changing these settings do not set the defaults for the control.
The defaults cannot be set and the font, font size and colour etc will need to be set at the time of the item creation using the formatting controls of the Multi Lines of Text control.