The little old man around the corner from me rescued a german shepherd last year. she was a tiny anxiuos thing and she has grown into a lovely girl but she is still a litle underweight although considering what she was like she is fantastic.
He has been spending aroung £60 a month on tinned food for her so i advised raw as its better and cheaper and as he has no transport it will be delivered.
I have said im happy to add his order to mine and he has asked me to decide what to get.
Now she is slightly smaller than average GSD although not a lot smaller, and she is quite an active and sometimes nervy thing which sometimes makes her drop weight.
how much should she be eating per day?

my guess would be 2.5 to 3 % of her body weight. If she is on the thin side you could go for 3%. The great thing about raw is that you can adjust it accordingly to the dogs weight. Hope this helps?

That sounds about what I feed my gsd % wise. I agree the beauty of raw is that you go by eye and the health of your dog just like you would with yourself and the family. 

As said, 2-3% depending on age/activity. If you wanting him/her to put on weight try more, or lose try less. See how they go and incress/reduce accordingly.
Here is a site to calcuate ammounts ...  http://www.raw4dogs.com/calculate.htm