Rightio, for anyone that might be interested in how to accomplish this, here is the quicker and much dirtier answer.....
No need to worry about changing the size of the frame itself. Since texture are not clipped to the frame and they can "live" outside of it, one solution is to have the frame the size of your portrait and then set textures for the fill and border. Simply then anchor the textures to the right side of the fontstring and hey presto. The frame then appears to change size in response to the width of the string ( the targets name in this case ) The only caveat is that the only part of the frame that is clickable is the portrait itself, which in and of itself is desirable is this case... less chance of a random, mid-battle misclick.
There we go, if you can't to do it properly... hack!
V