You'll look taller if you stand up straight.

站直身子，你会显得高些。

《牛津词典》

If you stand up straight, you'll give an impression of self-confidence, even if you're quaking in your boots.

只要你站直了，就会给人一种自信的印象，哪怕你正紧张得发抖。

《柯林斯英汉双解大词典》

When you feel stressed out, you can also try to sit up straight or stand tall.

当你感到压力大的时候，你也可以试着坐直或站直。

youdao

It's not really hard. You just make your bed neat, remember to stand up straight, and always answer every question with "Yes, Drill Sergeant."

VOA: standard.other

And you all always thank us, but we stand here, stand up straight, breathing in and out every day because of your support.

WHITEHOUSE: Remarks by the First Lady at a Campaign Event -- La Jolla, CA