Insulin is measured in units, abbreviated U (international units, previously abbreviated IU).
One unit of insulin is defined as the amount of insulin that will lower the blood glucose of a healthy 2 kg (4.4 lb) rabbit that has fasted for 24 hours to 2.5 mmol/l (45 mg/dl) within 5 hours...
Funny definition right!
Another standard definition is
The activity contained in 1/22 milligram of the international standard of zinc-insulin crystals.
Insulin solutions have different strengths but the most common type is 100 unites in 1 ml and therefore you need to use micro-syringes to adjust small number of units.