Along with CCA, a VERY important factor is reserve capacity (since Ah ratings seem to be nearly impossible to get). I'd go for 150 rc at a minimum.
Think of it this way: CCA tells you how much oomph is available in the cold. Ah or RC will tell you how long it will be available. If your battery is marginal, then the CCA will fade quickly - during the glow plug warm up - leaving you without sufficient amps to start the engine.
A 94R battery case will fit in the '03 Jetta, and it probably will get you more battery capacity as well. So what if it lops a little outside the official footprint - like the above posters have said, the more battery the better.
If you have a Sam's nearby (yuk - I hate recommending that company for anything, but it is the best battery and deal I found) check out the 49 (H8) batteries under the Duracell name (made by EastPenn, a very good battery company). H8 fits nicely in the Golf/Jetta chassis and you can select enough amps to fulfill your needs.