The weight will shrink substantially when roasted. You will need at least 8 oz per person cooked, thus get at least 10lbs. I'd get a 12-14lb roast. If there are leftovers, you can make wonderful sandwiches.
One thing I'd consider is if some of the people eating were kids, and how many side dishes you'll be serving with it. I bought a turkey based on the recommended amounts for Thanksgiving and ended up with almost half of it left over. I hadn't taken into consideration four of the ten people were little kids, and that we had ten different sides.
For DS, DH, and I, I usually cook a 2 pound roast, that leaves us a little leftover.
Five pounds would probably be enough if there are a lot of sides. The rule of thumb from the butcher is 1/2 to 3/4 lb per person, so that would be 4 1/2 to 6 3/4 lbs.
Depends on how well you want them done. Use a meat thermometer and take them out just before they hit the temperature you want, as they will continue to cook after you remove them from the oven.
It's about 20 minutes per pund for rare, 30 for well done as a good rule of thumb. that can vary by the size and shape of your roast, go a couple minutes less per pound for a tenderloin.