Thats like saying......I wanna a new car but dont know what.......any ideas anyone......lol. If I were you I'd go and do some wheel homework to see what size you want and what sizes fit the S3.......and what wheels your after. Lots of threads on here that give you that relevant info.
There's not just one size that fits, and it depends on where/how you want the wheel to sit in the arch.
You need to look for a wheel you like, then find out what fitments it's available in. Like the others have said, you want 5x112 pcd or you'll need adapters to fit. Then, depending on the diameter and width, you need to figure out the offset (ET) range that'll clear your strut on the inside and your arch on the outside. If the ET is low, the wheel will stick out too much, if it's high then you can get spacers to step it out a bit.
Once you've found the right wheel, you then need to look at what tyres you can fit (again looks vs fitting). A bigger wheel can be squeezed in if you stretch the tyres, for example.