
It depends on what you want teh dog to do with the bone.........
I wont give mine anything other than raw......if you want a recreational bone, ie to be chewed but not consumed, then basically anything fairly large (depends on the size of your dog really).....mine find lamb shanks last a whole day or more, but then i have fairly small/medium sized dogs, something like a GSD would be bored of/able to chew bits off of a lamb shank.
Go for something pretty big, and supervise your dog whilst he/she eats it, after a while you will get an idea of what kind of bone lasts longest, and is safest for your dog.
Be warned, marrow bones are very fatty and can cause the dog to get the runs if they arent used to it.
Em