Here's a breakdown:
* Big ka na raw: This is a combination of "big" (meaning "grown up"), "ka na" (meaning "you have"), and "raw" (a word used to indicate hearsay or something someone else has said). Together, it means "you've grown up (according to someone else)."
* sabi ni tito Jay: This translates to "Uncle Jay said."
So, the entire phrase means that someone is reporting that Uncle Jay said you've grown up.