Stock brake size on a V6 4mo is 312mm x 25mm vented on the fronts and rears are 256mm x 22mm vented. I think there's no law against how big but i do believe you aren't allowed to go smaller than stock. No point in going bigger discs unless you get bigger callipers and pads to accommodate them other wise its wasted.
I've got a big brembo kit on the front. Be warned that if your upping size its going to cost you specially if you get it from Australia. Mine was around 1.7k with rears landed from uk. Quote for the front was 4k over here.
Also might be worth doing a bit of research into bigger brakes as it can really hamper your performance if you go too big as your adding more rotating mass and bigger brakes wont actually make you stop any quicker if your all reading engaging ABS, It will reduce the chance of heat fade but that's only worried about on a track.
Heres a link to common upgrades.
VWVortex.com - The brakes of the MK4 platform - OEM and Upgrades
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