How big of a sirloin roast do you need for 12 people?

For 12 people, you typically need about 1 pound of sirloin roast per person, especially if it's the main dish. This means you'll want a roast that weighs around 10 to 12 pounds to ensure everyone has enough to eat, accounting for cooking shrinkage and leftovers. Adjust the size based on your guests' appetites and the presence of other dishes.