All types of screw heads and screwdrivers come in one of four sizes: No. 0 to No. 4, with 0 being the smallest and 4 the largest. The most common size is No. 2. It's vital to match the size of the screwdriver head or tip to the screw you're trying to turn. If you don't, the screwdriver may slip and strip the screw’s head, making the screw useless and difficult to remove.
The length of the screwdriver shank is another important variable. You can develop more torque with a long shank, but you often need a shorter one to fit inside the space in which you're working. Shank lengths vary from a few inches to more than a foot.