lists, lists, lists!
Not sure about how big the turkey needs to be but as for veg, my trick (I did Xmas lunch for the whole family for the first time last year!) was to calculate using, say 5 sprouts per adult, 3 per child, plus one serve, half a 4" carrot per person, imagine your spuds cut up into roasties and calculate say 3 roasties per spud, 4 roasties per man, 2-3 per woman, 1-2 per child. 2 chipolatas per person (buy ready wrapped with bacon!), 2-3 stuffing balls etc etc. You have to actually visualise a plate of food (maybe do a roast chicken soon to get an idea!).
Prepare the veggies the night before.
Remember to take the turkey out of the fridge early as it takes a lot less time to cook from room temperature. Don't stuff the turkey cavity, stuff the neck OR use stuffing balls which you back separately.
Make sure you have enough appropriately sized roasting dishes! You won't have any spare space!
Timing wise, I used the guide given in I think it was last year's free Asda mag and worked backwards. I wrote it all down on a piece of paper and stuck as rigidly to it as poss! The main thing is to make sure the bird is cooked. If it's ready 'too early', fear not- a couple of degrees cooler than ideal, esp with nice gravy over it (wrap well in foil)- it won't matter AND the stuff in the oven will cook a lot quicker once the bird is out.
Finally, have a glass of white to hand at all times!