grin/smile from ear to ear 
 
to look extremely happy
非常开心，笑得合不拢嘴
 
smile from ear to ear 的字面意思是“笑容从左耳到右耳”，那可不就是“笑得合不拢嘴”嘛！
 
"We've had a fantastic response!" he said, grinning from ear to ear. 
“我们收到的反馈极好，”他说着，笑得嘴都合不拢了。
 
When he heard he had won the prize, his smile spread from ear to ear.
他听到获奖的消息，笑得合不拢嘴。