Interesting cultural differences. In AmE, "stuffed" can be somewhat informal, but if used in the proper context, it's not out of place at, say, a dinner party or business meal. For example, if your host offers you more food and you're completely full, it wouldn't be out of place to say, "No thank you, I'm stuffed. Everything was so delicious, I ate far more than I usually do!" For some reason, saying "I'm full" often implies that you might have some room left for dessert, while "I'm stuffed" indicates to you host that you are completely finished with the meal.