The diameter of the contact lens depends upon the diameter of the cornea. Lens should be 2.0 mm larger than the diameter of the cornea. So you can see that the size of a contact lenses is different from that of a eyeglasses frame. You should buy the correct diameter contact lens for your eyes for too big or too small will cause problems.
The diameter in contact lenses means the size of the circle lenses. As we all know, the cornea, also known as the colored appearing part of the eye, that the lens sits on is about 11 mm. consequently, if you buy the contact lenses with 15.0 mm in diameter, then about 2 mm will extend on all sides of the cornea, which will help the lenses still fit your eyes and make you feel ok. As usual, when you go to shop to buy contact lenses, the experts will tell you the diameter which fit you and your eyes. Here I want to tell you that you had better not buy the contact lenses online, because you do not know the exact diameter your eyes need. If you buy the lenses which are smaller, then maybe it will be hard to put them in.
In fact, it means the size of the lens, not your eye size. The cornea, I mean the colored appearing part of the eye that the lens sits on is about 11.0mm. The 15.0mm would extend about 2.0mm on all sides of the cornea. However, the diameter of the circular contact lenses, I mean the size has nothing to do with the power of your contact lenses.