I grabbed an ARB off a 5 door golf, (for my 1.6 Ryder) and it was 21.5mm in diam, can't remember if it was off a GL or GTI though.
I would have prefered the 18mm one, as I wouldn't have had to faff about with making my poly bush holes bigger to fit on the ARB. But it makes a hell of a diference for cornering.