The common thought is the XL is the no regrets egg. If you think you need it you probably do, it can be run on smaller cooks but you can't make the large bigger. I regularly cook for two and I've cooked for up to 40. My normal cook is for 4 to 8 and it's just great for getting it all on in one go.
Hopefully this is what you are looking for
http://www.biggreenegg.co.uk/shopIf you click on each size tab on the page you get and idea what it will fit on. It's slightly optimistic but not a bad guide
Any questions let me know. My XL is one of the best things I've ever bought, actually had bought for me but you get the message.