"Standard" amount depends on locale and more on service plan. People in the south tend to expect bigger pulled pork sandwiches. 3 oz is a small sandwich for a hamburger bun, 4 oz a generous one, 6 oz a larger bun or half of a 12" French roll, and generally 1 oz per inch of French roll. I suggest using ice-cream scoops to keep service size consistent -- declasse with anything else, but helpful with pulled pork. Since the choice is pulled pork, any questions about class have already been answered.
One-quarter pound of cooked meat per person is too small for an adult crowd, Cub-Scouts maybe. Budget at least 6 oz per head even for church ladies, and 8 oz for firemen and athletes. I think I'd go between 7.5 oz for a high school group. For Boston butt, figure about 30% - 35% waste, and for whole shoulders figure 40% - 45% depending on leanness and the way you trim. This is for for meat which has not been "enhanced;" are neither generous portions, generous waste figures; but they are not stingy either.
If it's cost plus, I'd feel beyond safe with 210 pounds of raw butt, or 240 pounds of raw shoulder. If I were covering costs against a per-head fee, I'd be be comfortable with 190 and 220.
FWIW, you can save a little weight on the meat if you serve cole-slaw in the sandwich, rather than along side.
Pulled-pork parfait is a popular barbecue menu item now. If the deal hasn't been fully negotiated, or even if it has, you might want to consider this service option. Best served in clear glass or plastic.
Finally, bread choice is very important and will partially determine the amount of meat eaten. I suggest a selection of top-quality rolls.
Good luck with your first big 'Q,