18 inch was standard, I believe you could order 19 inch LMGT4's as an (expensive) option.
About the tyres, My wheels came with some no name budget tyres and after reading some of the tyre comments on here I was absolutely petrified of them when I first went out, especially as it was raining. I crept along at about 40 for 3 miles, didn't dare touch the throttle in case I went spinning out. Well, here I am some 2 years later with the same tyres and I haven't found a single ditch or even had a slide but then again I'm a very careful driver and with my limited ability wouldn't notice any difference even if I had £600 tyres per corner. However, if you're pushing the limits then get them changed for something better.
Pete