Way too may variables. I've seen 19" wheels on a jetta but I don't know the specific dimensions. If you stick with an oem 5x100 VW wheel, you should be able to fit most 18". But go too wide/tall on the tire and you'll get fender rubbing, possibly rubbing against the fuel tank neck, etc.
Also, check out this tire/wheel size calculator, it also shows differences in clearance.
You could keep the lighter 15" wheels to keep weight down and go with stiffer shocks. It would give you better handling while keeping mileage up. I don't think most people would even know that you had rims unless they were chrome blingers.
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