There are many different versions this is the most common kind It really depends on your preference for what it is you're wanting do you want one that takes triple-a batteries do you want 1 with an internal battery. Given each of the different ones Given different battery types will actually affect how big the sizes are. So just about how big of a thing do you want to lug around.

The second link is the one i use is a bit bigger (about 50cent piece size) allows the use of TWO accounts simultaneously but it takes batteries depending on how much use is it they last about 2 weeks with daily use.