Depending on which year you're claiming an ERC refund in, the requirements that determine small and large employers vary.
For 2020, small employers are those that had 100 or less full-time, W2 employees in 2019. Large employers are those who had 101 or more.
For 2021, a small employer is any business that had 500 or less full-time, W2 employees in 2019. A large employer is any business whose 2019 full-time, W2 employees exceeded that amount.