I oven roasted a 15.5lb, seven bone rib roast for Christmas Day dinner. It was longer than the roasting pan I would normally have used, so I roasted it in doubled up aluminum steamer pans. Roast was rubbed with gravy master than sprinkled with salt, pepper, onion and garlic powders with a little white wine in the bottom of the pan. It was placed in a 350 degree oven, which was eventually lowered to 300 degrees. My rule of thumb is to always pull the roast out of the oven after 2hr 30min, but since it was so large it was left it in until the 3hr mark then left to rest.
The result was a medium roast, the way my wife and I preffer, not a walking away rare piece of meat as my kids would have preffered.