You want the feed to the airbox to be bigger than the diameter of the hose from the airbox to the turbo. so you always have plenty of air in reserve.
I used 100mm (4") duct.
To fit that to an S3 you will need a FMIC though, as there is no room...but with a FMIC fitted you can run a 100mm duct from behind the front bumper to the airbox.
Where in the front bumper to take the air from?
Well if you are really sad you can measure the areas to determine which has the area of highest pressure (yes, I really am that sad) using a manometer and set the duct there.
To save you the work...I found that a 100mm duct facing down towards the floor in the area behind the passenger foglight, level with the top of the foglight grille to provide the highest positive pressure area.
It's only mb...but it helps.
Keeping the duct in the high pressure area, but NOT pointing forward at that low level reduces the chances of picking up water from puddles, and the associated (expensive) hydrolock issues.
Also, if you use a 100mm duct, get hold of a 100mm/4" subwoofer port to put into the end of the duct to smooth the airfolw into the hose...it also gives a firm place with which to mount the end.