We can't tell you what a good weight for your GSD is. There is no true weight guide as it depends on the individual. At 7 months, she may be going through another growth spurt and that usually makes them lanky. She will fill out. Being lean is better than being overweight. As long as she isn't emaciated (all ribs showing, hipbones protruding, loss of muscle mass), she will catch up when her growth slows down.
Xan Varik von Fernheim (born 4/3/13)