We found them to be good sized but not super large. To start we did the Greek Meatballs and there were 3, a salad that fed 3 of us and had some left over and we had some shish kabob platters that were nice sized.
Mercato is absolutely adorable. It's great for sipping wine and chatting...they do cocktails as well. Every time I'm there it's just for small bites- usually the mussesl which have been excellent. Not sure about the rest of the menu though.