There are some measurement which will decide whether the contacts fit your eyes or not.
Cornea measurements. An instrument called a keratometer will be used to measure the curvature of your eye's clear front surface.
Pupil and iris measurements. The size of your eye's pupil may also be measured. In a simple approach, a card or ruler showing different pupil sizes is held next to your eye to determine the best match.
Tear film evaluation. Contact lens fittings may also include a tear film evaluation.
Evaluation of your eye's surface and contact lens fit. The health of your cornea will be evaluated using a biomicroscope.
The contacts lens measurements include cornea, pupil and iris measurements, evaluations include tear film evaluation, eye's surface evaluation and contact lens fit evaluation. Don't worry. These steps will not hurt your eyes and it will not be very hurt. The doctor will perform this gently.
The doctor will use keratometer (a kind of instrument) to measure the curvature of the clear front surface of your eyes. This kind of instrument can help the doctor to choose the proper curve and size of your contacts. Sometimes, the doctor will also use wavefront measurements to identify higher-order aberrations of your eyes. This can help the doctor to choose the type of contact lenses which can give you a sharp vision.
My doctor asked me to sit on a chair. And then she moved a machine which was placed right in front of me. She asked me to place my chin on a support that was in front of the machine. Then she placed the machine right in front of my right eye. I heard a click. Then she told me not to move my head so she could check the other eye. After this, she asked me to wear a pair of strange glasses and told her the directions of the letters on the wall with one eye. And if i told her that i couldn't see it clearly, she would change lenses. After doing this with both eyes, she told me the degrees of my eyes.