on an svt the closest overall diameter to stock is a 215/40/18 that Julian recommended. I have also run a 125/35/18 but the speedometer will be a little off (it is also the closet overall diameter to stock for a standard mk1 focus)
I have 215/40 on right now.
'14 Tuxedo Black ST3
'13 Ingot Silver Escape Titanium AWD
'04 CD Silver SVT, #877, 1 of 135 May 2011 ROTM, FF11: Best Display, Best of Show; AFN12: Best SVT, Best Audio, Finest Five, Carlisle Nifty Fifty