Just give a quick “oh, thank you” in a pleasant, surprised tone. Comes off as gracious, but not conceited. Unless it’s something you can’t help, then it’s more charming if you act like it’s a skill in a cheeky way. I.e “wow you’re tall”

“Thanks I’ve been working on it”

It’s worth noting, you’ll come off as more awkward if you try to fire back a compliment yourself. It came organically to them, and isn’t necessary to mirror, unless you feel like they’re fishing.