How much should I eat for breakfast? What’s a “normal” portion?

It dependance on what you’re eating but normally pre dosed is okay. Let’s say you want to have Greek yogurt + granola. One cup of yogurt (around 150g) and a sprinkle of granola (around 30~50 g) are perfect to start the day off. If you’d like to eat something different (let’s say porridge or pancakes) I suggest you look online on forums or nutrisionists’ pages rather than yt videos

Measure your body composition if you have the opportunity. Depending on your objective. But if “normal”, you can try follow My Healthy Plate guidelines.