Hi Jeff,
My apologies for the delay in getting back to you.
If you intend to resize the symbols proportionally , then the Diagram symbols can be resized proportionally by stretching the corner selections with the resize handle.
If you want you can disable resizing of symbols using one of the dimensions ( Height/Width ) by overriding the DrawResizeHandle(..) method.
I have created a sample implementing this feature,kindly refer the attached sample .I hope this would help you to resize symbols as you wish.
Thanks for your patience.
Thanks & Regards,
Ramya.
ResizeProportional.zip