How big should the meatballs be - or how many should the recipe make??

Looking at the size of the orzo and the spoon in the photo, the meatballs appear to be pretty small, maybe an inch in diameter so bite-sized.

I don't see why you couldn't make them a little bigger if you prefer though.

Myself I would opt for the smaller size pictured but do what you think will please the people who will be joining you at your table.