What size alloys could i put on my skoda octavia without causeing any problems?

You have to provide more information, and maybe I can answer your question. I need to know the year of the car, the size of the engine and the trim levels. Keep in mind that just because the tire will fit, does not meet that the engine can handle the extra size. I had a Suzuki Samari that I was able to put 1100 Ground Hog tires on. I had to special order the rims (Ford rims ironically enough) and cut back some of the sheet metal.