Every meal type is customizable directly in the meal planner. Maybe you want to remove breakfast, or make snack meals smaller and have no cook time.
To start, click a meal's name. You'll see a menu with options to edit the current meal type, switch it with another meal type, or create a new meal type.
When editing or creating a meal type, you'll have the option to filter the foods by your available prep time, whether or not you want to cook, and whether or not you want breakfast foods. You can also choose whether or not you want to only use recurring foods for a given meal (this is a great way to have total control over the generator's results. Check this box and the generator will build the meal while selecting only from whatever foods you set to recur).
You'll also be able to set a meal size. The size is relative to the size of the other meals - the ratios on the sizes of tiny/small/normal/big/huge meals is 50/75/100/125/150. So a normal meal will usually be about twice as big as a tiny meal, and a huge meal will be about 3 times as big as a tiny meal. Because the meal sizes are relative, if you have a tiny meal and a bunch of normal size meals, but you want the tiny meal to be even smaller, change the normal size meals to huge meals.