The size of the beef will not vary the cooking time required for this recipe. I use a two bone fore rib of beef because it comfortably feeds 4 people. My butcher cuts the bone off so it does not poke through the top, if your joint has the bones showing, you'll want to wrap them in foil.
Before putting the beef in the oven you need to brown the outside. You could do this on the stovetop but I like to use my baking blowtorch to brown the beef.