What size is the iPhone SE SIM card?

The iPhone SE has a Nano SIM card (4FF).

Apple first utilized the Nano SIM form factor in the iPhone 5. Previously the iPhone 4 and iPhone 4s used a Micro SIM card.

By making the SIM card smaller, Apple engineers have more space to fit other hardware and minimize wasted space. Nano SIM cards are backward-compatible, and can be used in older devices with an adapter tray.