Had stomach pain they did ultra sound and said kidney was larger than normal. what can cause a large kidney? is that concerning?

Many things: Can do this, including size variation from person to person (and in the same person from side to side). If there is a concern that the kidney is enlarged as a result of a problem then further evaluation using MRI or ct is often performed. Also, birth anomalies, abnormal opposite kidney, infection, obstruction, and tumors can all do this.