What is the maximum size tyre/wheel that can fit onto an unaltered Fiat Punto mk2 3door 1.2 8v car?

Don't understand what you wantr to know this for.

Whatever size your wheels are relates to the correct tyre size to prevent your speedo being out.

If you mean what WHEEL size is a maximum then it's whatever you choose but I probably wouldn't go above 18's. Partiularly an unaltered 1.2 8v engine. Just watch it struggle and eat petrol

lots of peopl eon here have 17 on theres but it does effect acceleration