Some stylists prefer short scissors, others prefer long. To some extent it depends on the type of cutting you do. The length of a scissor is measured from the very tip of the blade to the end of the longest finger hole. It doesn't include the finger rest.
To find your ideal size, place a pair of scissors on the palm of your hand with the finger hole touching the base of your thumb, the tip of the blade should be in the last section of your middle finger.