Short grain rice releases its starch as it cooks, which is exactly what you need for a great risotto. Long-grain rice just won’t give you the same texture, no matter how much cooking and stirring you do.
The most common type of rice used in risotto is Arborio rice. It contains an extremely high level of starch. It's all that starch that makes the dish so creamy. It would make things easier if there was a good substitute for arborio rice—but there’s really not! This unique rice is what gives risotto al pomodoro its signature creaminess.
If you’re feeling fancy (and feel like spending a little more money), look for Carnaroli rice. Like Arborio, it has very high starch content. Carnaroli is considered the premiere rice for risotto. It’s not easy to find unless you are in Italy or an Italian market—but if you do find some, stock up!
Rarest of all, there’s also Vialone Nano rice. This one is grown in the same region as the Prosecco grape. Hard to track down, grown in small crops, and usually only found in Italy, this one is really special if you can get it.