What size should ground wire be?

Ground wire size follows the breaker protecting the circuit, per NEC Table 250.122. The ladder for copper: 14 AWG up to 15 amps, 12 for 20, 10 for 30 through 60, 8 for 100, and 6 for 200.

The ground can be smaller than the hots at higher amperages because it only carries current for the instant it takes to trip the breaker, not continuously.

A separate table, 250.66, governs the grounding electrode conductor that runs to your rods and water pipe; that one sizes from the service conductors instead.

One easy miss: upsize the circuit wires for voltage drop and 250.122(B) requires the ground to grow in proportion.

The ground wire size calculator reads both tables and labels which one applies to your case.