What size disc rotors should I put on the superb 2, there are x3 size options 288mm, 312mm and 345mm. Can I use any size?

More than likely 312 mm - that generally seems to be the case for the 125 kW 2.0. 345 mm was only fitted to the 3.6 V6 model. You must replace like with like. To be certain you will need the PR code for the front brakes: a Skoda dealer will be able to get this for you. Alternatively you can purchase a subscription for erWin and download the 'Vehicle Specific Information' for your car: an hour's subscription is pretty cheap (about €10 here) and you will need your VIN.

Or even measure the current discs

The 1.4 petrol comes with the 288MM front discs, the 3.6 petrol with the 345MM front discs. All the other cars have the 312MM front discs.

Some day half my posts will be about front brake disc sizes.

Thanks thought as much.