Small portions? Try the Zuppa di Pesce. My wife and I always take half home and split it for lunch the next day. I've never seen so much seafood in a bowl for one.
The food is excellent, and offers Boston diners a more Italian dining experience than what they would typically find in the North End. This explains both the ban on tap water and what Annabella found as small portions. Perhaps a request for toasted bread crumbs on the seafood might fall favorably on the ears of the chef, although I have not tried this.
He's really a jovial sort who puts up with my feeble Italian (and really appreciates any attempt). The first time we went, a small glass of peach grappa came with the check for each of us. I keep forgetting to ask him where I can find some of that--although my wife's suspicion is that it was homemade.
They expanded the restaurant into the space next door a few months back, so there is a lot more seating. This hasn't affected the service as far as we have seen. One side affect is that we do see less of the chef than we used to, but the new staff are quite competent and helpful. We've never had to wait when we made reservations.
The desserts are typically Italian, in that most little trattorias and such in Italy serve premade confections from Bindi. They have a nice selection at Vecchia Roma, but someone at the table should get the tiramisu--it's homemade.
If you have to go to the North End, try the Maloreddus at Maurizio's. But you don't have to go that far.