I think it probably depends on the restaurant and what you order. I think any appetizer with expensive meat/seafood tend to be smaller. We've had some pretty large appetizers though.
The flatbreads at California Grill are definitely shareable (we shared between two of us and still took some home). I think some of the sushi choices and the cheese plate are also large enough to feed more than one person.
At LeCellier GF had the mussels--a pretty big bowl of mussels and she started getting full before the entree even came. I had the tomato stack and that was not quite as shareable or filling.
At Flying Fish I had a huge salad (it was a peach/gorgonzola/pecan salad I think)--just a huge bowl that seemed to go on forever. I had to force myself to stop eating it so I had room for the next course.