Answer
Standard tyres are 205/55x16 and if you wanna
keep the rolling diameter exactly the same then you would have to buy 215/45x17 or 225/35x18 tyres. i believe that 18 inch wheels with 225/35x18 tyres would be the largest ones that could be fitted.
if you want a great forum for info of skodas then have a look at www.briskoda.net
cheers