i take it its flashing when just driving along normally and not booting it about in the wet? lol.

standard size on 16's is 205/50.

presumably if the tc system is using the rear wheels as a reference for actual vehilce speed then by having smaller tyres on the front will make the fronts spin faster than the rear (conversly the rears spin slower as its the fronts that are driven) which could confuse the system.

only way to check for sure though is fit another set of wheels with same size tyres all round. got any friends with focus'?

if you have standard brakes on then you can fit any mk1 focus wheels too