Point of Order: Unless you count Poussins... They are about a mouthful each...
Like people have said, look for a chicken labelled Medium, or maybe 3 and a bit pounds in weight. Roast according to instructions on label or what ever your sister said. Usually it's in the form of X minutes per pound or kilo, plus X minutes. Any meat not eaten on the day can be stripped off the bone, kept in the fridge and used to make sandwiches, salads, in with pasta, risotto...