Hello!
Yes, you can say "You'll do great!". Other ways to let a person know they'll do a good job and you wish them luck are:
- Good luck.
- It will be fine / okay / great / amazing.
- I believe in you.
- You can make it / You can do it.
- You will kill it.
- Last of all, the idiomatic expression "break a leg" is also used to wish someone good luck.
I hope to have helped you!