It comes with a key chain for your convenience
It measures approximately 1 5/8" (41 mm) long and 9/16" (14.5 mm) in diameter
I've used these microscope/magnifiers before and have been very happy with them. They are the clearest with a good focal range (others I've used may have had greater magnification but were often blurry or had a very short focal range so difficult to keep in focus). I've searched many sites for this particular microscope and this website seems to have the best price (only a few days for delivery too). Highly recommend both (lens and website).
Hint: I don't use the keychain so I took it off and I took off the bottom clear plastic collar to get much closer to the thing I'm viewing (circuit boards mostly).