The cone must be that large that the dog can not reach and scratch himself.
Here are few pics:
This dogs cone is too big.
http://hearingelmo.files.wordpress.c...6/dog-cone.jpg
This one is too small.
http://t3.gstatic.com/images?q=tbn:A...tLkVCffdUys1IQ
The head sticks up from the cone it should not do that.
http://www.ibirite.net/wp-content/up...%20small_2.jpg
This one is the right size.
Just watch out that the nose doesn't stick out of the cone then he can scratch himself most likely
