AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.