Is a liver size of 15 CM on ultrasound enlarged for a 5'4" 138 lb 20 y old female? Ultrasound says "negative right upper quadrant ultrasound."

Just right. That sounds about the right dimension seen of the liver. Pay attention more to things like fatty infiltration or densities. It seems there was a concentration on RUQ like for the gallbladder but it seems there is nothing to worry about. Just keep it healthy now.