留学生在形容眼睛很大时，可以使用以下表达方式：
网络流行语
眼睛好像两个熠熠生辉的水晶球，明亮而清晰。
眼神像无尽星海，深邃而引人遐想。
眼睛好像两个大灯笼，闪烁着光芒。
传统形容词
皓齿明眸，双瞳剪水，炯炯有神。
明眸善睐，眉清目秀，眉目如画。
英文表达
A pair of bright, beautiful eyes。
Eyes that shine or sparkle。
幽默风趣
Your eyes are so big, you could probably see into the future.
Your eyes are bigger than the moon, you should consider becoming a satellite!
比喻
Eyes like the sky, clear and bright.
Eyes as deep as the sea, playing with light and shadow