Usually for a roast spit it does not go by age, it goes by weight. some spits can take 60-70 kilos, the bigger the spit the bigger you can have you carcase. I would not freeze the whole carcase, just think how long it would need to defrost. If your pig is ready weeks before it is needed I would seriously think of buying a carcase from the slaughterhouse you use. If it is ready a couple of weeks before you need it, you could ask the slaughterhouse to keep it for you till you need it, there may be a charge for your carcase being kept in the chiller. I am unsure how long you can actually hang a pig for but I will find out for you.