1x1 pressure grids are extremely laggy. Probably at least half of users would have severe issues running TPT if air was 1x1 in a save. 1x1 is 16x as laggy as 4x4. 2x2 is only 4x as laggy so probably a lot more could run it.
I'd rather not allow customizing this, it would make the code messy, and break a lot of conventions we have everywhere where we assume air / walls / other grids are 4x4.