The size of winch is dependent on the size of your vehicle and how much weight it will need to pull.  As a guide it should be between 1,500 and 5,0000 pounds but to be sure, always go for an ATV winch that can pull at least 1.5 times the maximum loaded weight of your four-wheel drive. The typical terrain can also be a factor to consider, eg. marshy, as you may need extra pulling power on top.