At Christmas you usually want a big-enough turkey to provide leftovers - but not to go to waste. Here are some guidelines. These weights are for uncooked turkeys.
Count the number of people who will be eating turkey - remember that this number may be different than the total number of guests if the group includes vegetarians.
Count for 450g (1 pound) of unstuffed, whole turkey per person for moderate leftovers. Allow about 600g (1.3 pounds approx) per person for more leftovers.
Figure on about 400g approx (3/4 pound) per person if you're buying just a turkey breast. Allow 225g (1/2 pound) per person for boneless turkey breast.
Allow about 500g (1-1/4 pounds) per person if you're buying a pre-stuffed, frozen bird.
Remember that you can find turkeys as small as a few pounds and as large as nearly 18kg (40 pounds) - and that the larger the bird, the higher the proportion of meat to bone. If you order a larger bird, you can freeze cooked parts for later.