if your using a type 9 'box, and sticking with the x/flow flywheel, your limited to 190mm clutches, if you use a pinto flywheel you can use a 216mm clutch, the zetec clutch is 220mm. if your using a T5 'box then you can use a 2WD Cosworth clutch which is 240mm
basically the bigger diameter clutch the better as it has a greater surface area, call Dunnell as they do a lot of RWD Zetec conversions