We generally used 4 oz fresh as the standard, simply because most fresh pasta recipes, made in bulk, break down to 1 1/4 LB, (or at least they did when we were doing them) however, you can use whichever size of portion you wish. Keep in mind whether it's a starter (antipasti or antipasto), primo or secondo the portion sizes will smaller or larger.
If "fresh pasta" is the highlight point of the dish, meaning the main portion, (which is what it seems to be) I would possibly go with 6, or even 8oz portions depending on if the dish is straight pasta or if it has some other item with it.
Make all of the dishes on the menu in different sizes before you open, and ask yourself "would I pay for this" and be objective about it, if you wouldn't or you would be disappointed, add a bit more.
This is the best advice that I can give.