To edit a meal's preferences, click the 3-dot menu button next to that meal type in your plan and select "Edit meal settings." (Alternatively, expand the menu at the left sidebar of the site, click "Meals & Schedule" and then "Meal Settings." You can edit each meal type from the "Meal types" section or create new ones here.)
One of the preferences in the meal settings allows you to change the size of a meal. You can choose between tiny, small, normal, big, or huge meal.
These are pretty ambiguous names for meal size, but here's how it works:
Each meal's size is relative to the other meals in the plan. Tiny meals have 50% calories, Small has 75%, normal has 100%, big has 125%, and huge has 150%.
So if you ask for 2000 calories, 4 meals, and all of your meals are the "Normal" size, all of your meals should be about the same number of calories. If you change one meal to "tiny", it should be about half the calories of a "normal" meal. If you change all of the meals to "tiny", they'll all be about the same size again, and the same as if they're all "normal".
If you change 3 out of 4 meals to "Huge" and the last meal to "tiny", the tiny meal should be about 1/3 the size of the rest of the meals (50% is 1/3 of 150%). If you have 1 tiny meal, 1 normal meal, and 1 huge meal, the tiny should be about half the size of the normal meal, and a third the size of the huge meal, and the huge meal should be about 50% larger than the normal meal. The actual size is determined by the number of calories you enter, and the meals simply divvy up the calories based on their size.
Note: Setting the meal size to "tiny" will also force the generator to use a single recipe in that meal (same with setting a "no time" restriction on your cook time). If you set all of your meals to "tiny", this can be used as a workaround for getting meals with just one dish in them.