nysauce,
The answer to your question of course turns on what kind of NY style pizza you are talking about in terms of toppings, kinds of cheeses (whole milk, part-skim), etc., but I have read that a single slice of pepperoni pizza out of a 14-inch pie that uses a hand-tossed dough has anywhere from 230 to 280 calories. Those numbers presuppose 8 slices to the 14-inch pie. Since an 18-inch pie has a surface area about 40% greater than a 14-inch pie, if my math is right the number of calories in a single slice (1/8th of the 18-inch pepperoni pie) goes to about 320 to 390 calories (and maybe a bit more because of the proportionately larger rim).
For purposes of your diet, would you like the numbers for a 1-inch pepperoni pie

?
Peter