Well, it is one of the only sources available for that kind of stats. It's not very accurate like you said, but there aren't many (or any) other services like it that I know of.
Anyways, I don't think there is a certain defenition of your "site size." It is usually what you think of your site, and from my perspective I think your site is somewhere in the top 20 percentile of popularity and size. It is really hard to tell though, because it depends on so many things. Some people compare the size of sites based on the ones they have made, and some base their personal decisions on sites such as Google and so forth.
Anyways, I think your site is quite good and popular as well. Look at your hits and keep trying to make them greater as time passes on. That's the best you can do from what I know.
