When someone hands you a plate at a fiesta, it’s fine — expected, even — to say “oh, I’m okay” the first time. Let them offer again. By the second or third offer, accept graciously and eat with enthusiasm; declining every time, or letting it drag out too long, is its own kind of rude.
If you’re genuinely full, say so warmly but still take a small portion and eat some of it. And don’t be surprised when your host insists on wrapping food for you to take home (balutan) — accept it. Turning down that offer, not the first plate, is the real faux pas.