What is The biggest tire size that will fit on a 2000 gls Jetta?

It all depends on what size wheel you are trying to use. On my track car I have 265/45 R16 slicks that fit fine on the car at stock ride height with 16" x 9" et20 offset wheels. Edit your post with what size and specs of your wheels and whether you're trying to maximize width, height, or both, and I'll let you know what will fit with the stock suspension.