It ranges all the way from California state senators (930,000 people per district, more even than a House seat) to New Hampshire state reps (3,000 people).
A lot of the flipped seats, in both of those figures, are in New Hampshire, which has a very big and very swingy legislature.
For example: losing the Kentucky House in 2016 was a big blow for Democrats. Gaining the New York Senate in 2018 was a big victory.