Whats the largest wheel size you can fit on a 2006 jeep grand Cherokee?

I'd recommend a site such as JeepsUnlimited.com for more info about doing aftermarket things to your Jeep. Theres a ton of forum and tech info there that will help you make determinations as to how stupid you really want to get in adding lift or lowering. Anything bad idea you can come up with has already probably been thought of and tried and posted about on a site like that and that would be your best source of info.

Tirerack.com is a great resource for learning about all the intricasies of tire/wheel size and offers a wide choice of tires/wheels to pick and choose from.

Goodluck in making your Jeep stand tall !!!