I tend to like 3m by 3m or there abouts, I can live with slightly less width but like the length - can store lots of gear underneath. They certainly do a nice selection of tarps there, but I personally really don't like that type of traditional tarp material for using out and about. I find it slightly bulky and noisy. Of course depends on what you are going to use it for though.
Sizes are an odd thing and personal, however more focused on what you are doing and what with. Many people like the smaller tarps 1.7 x 2.5 or something like that, enough to sleep under and not much less. Many others like bigger, as it allows you more potential configurations thus more freedom in your set up.
What are you planning to do with yours, etc. ?
Nag.