How much should one eat, for it to be considered a decent breakfast?

I try to at least get a balance of protein, healthy carbs, and a fruit or vegetable while mindfully eating. This doesn’t mean 30 grams of protein if I’m not super hungry. I don’t like to force an amount of food. Breakfast some days could be whole grain toast with peanut butter and banana. Oh, and always coffee. I like the ritual of making it in my stovetop espresso maker.