system
A chat on subreddit /r/NoStupidQuestions
human
Maybe have taxation start after a certain revenue threshold? I've always thought that makes sense. Keep tax exemptions to protect smaller churches, but there's no reason a Mega church can't afford to put some money in the pot.
gpt
That or go by attendance numbers.

For example, the law could define a megachurch as one where, in the past year, the church had, on average, more people (other than church leadership) attending church in the US than 10% of all churches in the US. 

Or - a more refined definition - the law could define a megachurch as one where the church had, on average, more people (other than church leadership) regularly attending church in the US than 10% of all churches in the US. A person is considered to be attending regularly if they attended at least 26 worship services in the past year.