I was always told a good rule of thumb for choosing a winch is it should be rated for at least 1.5 times the weight of the truck it's attached to, I.E. an 8000 lb diesel truck should have a minimum 12,000 LB winch.
If a straight line pull won't do it, simply use a snatch block at the anchor point and bring the winch hook back to the winch vehicle. The pulling speed will be cut in half but the pulling power will be doubled!
As Bobby said, you can see pretty quickly how easy it is to overload your recovery gear. With the use of a snatch block, several tree savers, or whatever you use for your anchor point, may need to be used. -