You can set the height and width in the href call for ‘TB_inline’ for both the private and public message boxes.
For even more control, via css, you may be interested in
http://www.philopress.com/products/buddymessageux/
That worked perfect, thank you! For anyone else reading, the dimensions that removed the scroll bar on the right, were;
Private Message Height – 335
Public Message Height – 255
On Safari 6.1.1
Thanks again!