I worked this out by hand a few weeks ago because my LoseIt app has a figure for sushi rolls too, but there is no way of knowing how big the rolls were that they used as their standard, and I know I can often cheat and make mine bigger than restaurant ones if I don't portion carefully.
Can you estimate how many cup measures the rice took up? Imagine the total size of the roll compared to the size of a tennis ball or your fist, if you don't usually use cup measures. White short-grain rice, cooked, rates at 267 calories per cup according to CalorieKing.com and a couple of other calorie sites I checked. When I make my own sushi, I usually figure calories from dry rice, 175 cal for 1/4 cup according to the brand I use. I think this is more accurate, because I know exactly the rice I used, without a possibly variable amount of water affecting the cooked size. The rice generally swells about 2x-3x size with water when cooked; this should make 1c cooked rice 233-267 cal. In sushi the rice is packed pretty tight so the 267-cal figure could probably apply.
The rice will be the majority of the calories in most rolls, unless the roller is like me and talented at stuffing, oh, a good quarter of an avocado into each roll.

Is the avocado visible in the roll closer to 1/4" across or 1/2"? If the latter, it's a 1/4 avocado or 70-80 cal.
Normally a small amount of sweetener (sugar or mirin) and vinegar are added to the rice which would account for maybe a half teaspoon sugar (tops) per roll. So I would figure for a roll containing one cup rice with generous avocado, about 350 cal. Most traditional Japanese sushi chefs in restaurants that are not very Americanized DO make them much smaller than that, but "American-style" can be big, as ValRock noted above.
The What's Cooking America web site has a sushi tutorial which estimates calories for homemade rolls. But the math doesn't seem to work out there. Their instructions say use "3/4 to 1 cup of rice" per roll and then give a count of 246 cal for the avocado roll, 136 for the cucumber roll. In the second one the figure seems a bit low if they are really using the suggested amount of rice. (The total for that roll will basically be the calories of the rice, as the seaweed wrapper and cucumber have a negligible amount of calories.)
What's Cooking America Sushi Tutorial
So all that probably didn't help you
at all, except to suggest that maybe the figure you have could be for a roll with a fairly small amount of rice, maybe 1/2 to 2/3 cup.