Well, that depends on the size of your other meals (and drinks) you consume during the day.
Generally speaking, shoot for eating 2000 calories. How you divide this up is left to yourself, although the more veggies you can eat, the better (and they have very few calories, so you feel more full).
If you have a light breakfast (400 calories, two eggs and toast and a cup of coffee roughly), a medium lunch (500 calories, a tuna sandwich and water), and a heavy dinner (900 calories – a hamburger and fries/salad, a can of pop, and a few cookies), this is roughly 2000.
You could also do 6 snacks every 3 hours, about 330 calories each with water to drink. Or two meals at 900 with a soda to drink.
Realistically, the right sized meal for you is whatever you can eat SLOWLY before you are full. I say slowly because it takes time for your stomach to tell your brain you are full (10–20 minutes).
Of course, if you are still growing, your body will need more food. And your exact calorie requirement differs depending on your level of activity (if you’re a pro boxer, you are burning much more calories than a desk worker). Here’s a calorie estimater which will tell you approximately how many calories you need to eat each day. You don’t really need to care about the weight loss aspect of that, although it’s good to note that you lose weight by cutting out 3500 calories, and you gain weight by adding on that same amount.