A typical tri tip roast typically weighs between 1 and 3 pounds.  Depending on the size of the roast, how many people you are serving, and how much each person wants, you can plan to serve about 3/4 to one pound of tri tip roast per person.  If you're looking to reduce the amount of meat served per person, a smaller roast is recommended.  For more generous portion size, a larger roast can be used.  For an average-sized 3-lb roast, it should feed around 8 people with generous servings.