Personally I prefer the 18x24 or the 24x36. You have to take into consideration what size your work space is, whether or not you need to store it, storage space, etc.
I also like an even smaller one or the 12x12 turning one to take to classes. They're about the same as rulers, bigger ones are great but so are the smaller ones depending on where you're using them and the size of what you're working with.
As long as it's big enough to do what you want and small enough not to be clumsy you should be fine.