For bakkies, it's simple. Take the gross vehicle weight rating (GVM) and multiply it by 1.5. For example, if you've have a 2011 onwards Hilux Double Cab 4x4 (standard). This bakkie weights 2740kg . Multiply this by 1.5 and you'll get your minimum capacity. In this case, it's a 4500kg (10 000LB) winch. A WARN 10 000LB is a good place to start, but for some extra grunt, you can move up to a 12,000LB. Remember, you can always go up in capacity.
On the powersports side of things, it's slightly different. If you have a sport quad or a smaller quad (under 300cc), a 1,500 lb. winch, such as the XT or RT15, is a good winch to get. If you've got a larger quad (300cc or greater in size), then you'll want to increase up to a 2,500 lb. winch, such as the RT or XT25. For the biggest quads, a 3,000 lb. winch (XT or RT30) is the way to go. Now, if you've got a Side by Side, you'll likely want the RT or XT40 with its 4,000 lb. capacity specifically designed to pull the extra heft of a Side by Side.
One note for powersports buyers: You'll have a choice between steel cable and synthetic rope. Keep in mind steel cable is better for utility tasks and for winching in abrasive situations that might be hard on synthetic. However, synthetic is lightweight, easy to handle, and is great for use in mud.
If you're looking for industrial applications, it's best to contact FarmAbility to be sure you're getting the right winch for your application.