There are a number of sites you can check how you rank on twitter.
http://twitterholic.com/ shows you how you rank by followers and in your location
http://twittercounter.com/ also gives you a twitter rank
http://twitter.grader.com/ gives you a twitter rank in the form of a grade out of 100%
Each site explains more about how their ranking system works, but it can get complicated and confusing since you might get a slightly different rank from each site, but it'll be in the ball park anyway :)