Why the hypodermic needle has different sizes?

The hypodermic needle has different needle gauge(Thickness),needle hub and needle length.

Needle Gauge/Thickness:

The gauge (abbreviated as “G”) of a needle refers to the size of the needle’s outer diameter. The higher the gauge, the smaller the diameter and the thinner it appears. Thinner needles typically correlate with increased patient comfort and less bleeding. 

Needle gauges can range from 7G to 33G while the standard range is between 18G to 27G.

The thickness of a needle is not to be determined by a patient's comfort level but rather depends on the medication that is to be administered. The thicker the medication, the smaller the gauge must be to ensure the medication can easily pass through the needle into the subject. One must choose a shorter needle if administering a subcutaneous injection and a long needle will be optimal for intramuscular injection.

Needle Hub

ISO sets the stands for needle HUB. It has issued a color scheme for needles that assists clinicians identify needle gauges simply by the color of the needle HUB. Below is the full-color scheme that all manufacturers with ISO certification use when manufacturing needle hubs.