no idea about what size you should get sorry.
i normally buy mine from sainsburys and get a free range bronze. they are very tasty.
you don't HAVE to stuff the turkey-i never do. i make stuffing seperately. i make it by opening a packet of stuffing, mixing with hot water and then adding sausagemeat.
cooking the turkey-get it out of the packet, give it a really good wipe all over and inside with kitchen paper.
i normally shove a big onion and some bay leaves and peppercorns inside it. then smear loads of butter all over it. then put some strips of streaky over the breast so it's toattaly covered. salt and papper of course. then make like a small tent out of foil for the turkey-making sure there is loads of roomat the top-then bung it in the oven.
bread sauce is like having a small vomit on the side of your plate so i don't make it.
i would strongly reccommend that yo buy delias christmas book. it has step by step guide to cooking the dinner and getting everything prepared-i follow it every year-really helpful. it also tells you waht kind of size to buy-how long it will take to cook etc