It's been a long time since my JK was stock and I have a friend wanting to off road her 2017 JKU on beginner trails but she wants a bigger more aggressive tire. How big can she go without running into any issues?

 

Let's see if this helps: First things first. Jeeps come stock with 3 different tire sizes:
1. 225/75/16 (29”x9”)
2. 255/75/17 (32”x10”)
3. 255/70/18 (32”x10”).

The backspacing on stock JK wheels is 6.25". That means that the mounting surface of the wheel is 6.25” from the rear lip of the wheel. The larger the backspace number, the further under the fender (and closer to suspension components and airdam) the wheel will be.
33x10 tire (also called a “pizza cutter”). An example is the 255/80/17 BFG KM2. Because this tire is the same width as most stock JK tires, there is no issue on road, despite the larger diameter. You can run this size without issue.
Now, let’s consider some tires that could potentially cause slight issues on road; the 33x11s. The common sizes of these would be 285/75/16, 285/70/17, and 275/70/18.