Well on GSF the biggest one for a mk2 Golf they have is 59-5 - so I'm guessing that would be the one.
Cheers for that, I've emailed GSF and they mentioned a 52/54 mm clamp, which I was going to order till I saw this and also on their website they mention "SHORT PIPE 54.5mm" for the 8v GTi and other lower models, so I'm guessing what they told me was incorrect (no suprise there!).