Syringes for insulin users are designed for standard U-100 insulin. The dilution of insulin is such that 1 mL of insulin fluid has 100 standard units of insulin. Since insulin vials are typically 10 mL, each vial has 1000 units. Choose the smallest syringe that big enough to hold the largest dose you take in a day.
The smaller the syringe, the easier it is to read the markings and draw up an accurate dose. If your largest dose is close to the syringe maximum capacity, you might want to buy the next size up to handle any increases in your dose adjustments.
For example, if your dosage is 29 units and you buy a 30 units syringe, you won't be able to use those syringes if your doctor increases your dosage to 31 units. 30 units syringes are available with two different kinds of barrels: one with dosage markings at every unit and one with dosage markings at every half-unit.
People who take whole unit insulin doses of less than 30 units should use the syringe with markings at every unit, because its large, easy-to-read markings make it simple to draw up an accurate dose.
People who take very small doses (such as children) and who are told to measure their doses in half units (such as 2 units or 5 units) should use the syringe with dosage markings at every half unit.