How far can you lower the b5 with 19 inch rims on it?

 

haha not far at all with out rollling of the fender

 

That has little to do with it. When you get 19" wheels you make the sidewall profile smaller so the actual outer diameter, and rolling distance of the tire is within 1 or 2% of what it was from the factory. Otherwise your speedo would be off by a stupid ammount. The issue with rubbing is lowering the car with wider tires, or a different offsett. Lots of people are lowered on 19" wheels its just a whole lot easier to bend a wheel, logic would say they are heavyer, and ride quality is decreased alot. If you dont wanna rub dont go wider than 235mm and stick to an offsett like 35mm. Im running 18x8.5 with 235/40/R18 and a 35mm offsett and i have minimal rubbing.

Bigger wheel, less sidewall. The outer diameter should be as close to stock as is possible.

Bottom line, get coilovers so you can adjust it how you want. You can lower the car just as much as you could with 18" or 17" wheels. The narrower the tire the lower you can go.