My 17s are 245-40's. They are very close to the stock diameter.. The FAQ says overall diameter is 24.71" compared to 24.68" for stock (rear).
I think if weight is really going to make a big difference, I will just pull the passenger seat next time. That's kinda close to the weight the 17" wheels added. Definitely faster and easier to do than pulling all four tires.. ugh.