An American girl doll is 18 inches high so a quilt about 24 X 24 would be perfect. I would just cut 4 1/2 inch squares and let her sew them together. Any doll quilt I have made for my granddaughters, I made just like a regular quilt but I used a thin, lightweight batting in them. The kids loved to chose their own fabrics and help sew them together. She will enjoy the time you spent with her.