I had a big GSD mix that developed osteosarcoma at the age of nine. It was in his left front leg. Because he was in such good health otherwise, I made the decision (with encouragement from the vet ) to amputate his front leg. Amputation took place just before 4 July. I ended up putting him to sleep September 12, 2001. Although he got along okay without his front leg, (back leg is definitely easier though) it didn't seem to buy him any time at all. In the week before I put him to sleep, it became clear by his fever and general disposition that the cancer had spread. He did okay on temporary steroids but when we stopped, the look on his face told me the end wasn't far away and I didn't want to wait until he was suffering like that again.
At the time I had three very small children and I did not have the money to spend on chemotherapy, nor did I think that my dog would've enjoyed spending eight hours a day at the vet once a week for treatment. Looking back, I would not have made him suffer through the amputation again. It only bought him about two months. And those two months were agony for me, although I'm not sure how it was for him. I do believe that he wondered where his leg went, although animals are so much more stoic and don't see things the way humans do. I think the smaller the dog, the easier it is, and he stood almost 30 inches at the withers and weighed a hundred and 21 pounds.
I wish you the best of luck.