float sourceWidth = 1024f;
float sourceHeight = 768f;
float m_fScaleWidth;
float m_fScaleHeight;
void OnGUI()
{
m_fScaleWidth = Screen.width / sourceWidth;
m_fScaleHeight = Screen.height / sourceHeight;
GUI.matrix = Matrix4x4.TRS(new Vector3(0, 0, 0), Quaternion.identity,new Vector3(m_fScaleWidth, m_fScaleHeight, 1));
GUI.Button(new Rect(0, 0, 1024, 768), "");
}