Grab a homebrew, this gets geeky... But you asked...
Newton's Law of Cooling is part of the answer. Given that you also know the amount of time you want to achieve your temperature transition from hot to cold (t), your overall heat transfer coefficient referenced to the external area of your chiller (Uo), your coolant mass flow rate (m'), and the amount of heat you need to dissipate (Q), then your external chiller area (A) is:
A = -m'c/Uo * ln(-Q/m'ct(LMTD) + 1)
where c is the heat capacity of the coolant (presumably water) and LMTD is the log mean temperature difference between the wort temp and the coolant entrance temp.
ooooooooo, must drink beer. Getting geeked.
Note: always, always, always check my math.