The factory bar is 12mm. That's a bit bigger than 3/8"(9.5mm) and a lot closer to 1/2". It does work, and it's way better than nothing. I had this bar on my old Spyder and ran it for a long time.
Carlos, you'll need to bend it a little to clear the frame. If you don't want to bend it, make some bushings and attach it like a proper swaybar instead of VW's weird floating method. I'll bet that would work as well as my cobbed-together custom bar.
IMHO the aftermarket 19mm is a little bit much, I had one for a short while.
I made a custom 15mm bar from an old 911 through-body bar with bushings and end links(for those of you that don't know). I think it's just about perfect on a swing axle Spyder.