A single Chateaubriand joint will average out at 3lb (1.36kg) this size piece will serve 6 people, or you could be generous with 4 people.
Remember to seal the meat on top of the stove before cooking. A piece of meat 3lb (1.36kg) in weight will take no more than 45 min at 230c gas 8 to reach med/rare.
Try brushing with honey before you place the joint in the oven. This will give you a black rim to your beef as you slice through.