Ok I want to talk pizza as I love pizza but it is something I try to watch carefully. I'm also a bit of a pizza nerd.
Domino's pizzas are as follows: Large: 1933 kcal, medium 1373 kcal, small 922 kcal, personal 526 kcal.
This is for the basic cheese and tomato pizza. No stuffed crust, no greasy meats, no sugary BBQ sauce base. This is about as healthy as it gets. A small pizza is easily a good portion of the calories you would get from meals, the rest being snacks, so I hope you skipped the other two meals.
As for oven pizzas, most range from around 700 kcal (tesco spinach and ricotta) to about 1100 (the more bready or larger ones). Some of the pizzas marketed as being 'take away style' are particularly large and calorific, such as the 'Chicago Town Takeaway Large Stuffed Cheese Pizza' which tops in at a whopping 1740 kcal.
So the very healthiest oven pizzas are not too bad for a meal, just as long as they aren't being served with chips or coke. The others should be eaten in halves or quarters.
As for proper Neapolitan style pizza, a very standard dough ball for a 12 inch pizza is 230g. This equates to around 137g of flour, clocking in at 492 kcal. Add in a 150g ball of mozzerella (the size of the small supermarket balls) and you add 354 kcal, making a total of 846 kcal, again excluding sauce, oil, toppings etc.
In short, a few slices of pizza is fine, but if you're eating a whole one, the chances are very very good its a calorie bomb.