there is a nasty formula its based on flow rates in gph and surface area of 1' of the coil and surface area of the medium, i said its nasty right
you can find it in 1970's waste water treatment data. but it comes out to about the same surface area as 2' of the coil length, which would be like 1 bio ball.
[edited added]
but thats a minimum. and a few bio balls are better