Perfect table size for a cruise

I've been writing a guide of top tips for first time cruisers for awhile now but it occurred to me earlier that I'd never really discussed how to pick a table size on your cruise.
For anyone that has yet to cruise, (and what's taking you so long may I ask), when you book a cruise there will be two main areas you can eat onboard. There will be the self service restaurant or buffet where you can sit where you want and dine any time you want and then there will be the main dining room.
The main dining room will normally have two dinner sittings of an evening, first dining from around 6:00 - 6:30 and second dining around 8:00 to 8:30. If you choose to dine of an evening in the main dining room then you will need to choose your table size and with a couple of exceptions this will be where you dine for the entire cruise. I'll address the exceptions at the end of this post but for now it's best to remember that you need to pick your table size when you book your cruise and each choice has it's own pros and cons.