I've been fourwheeling for 30 years and have 3 vehicles in my garage with winches on them. I remember when everybody would recommend the winch be rated at 1.5 times the vehicle weight. I have a Chevy K5 with 1-ton axles w/ lockers, 40" tires, full cage, bumpers, blah, blah, blah that weights 6,000 lbs. with a 9,000 lb. winch. I have always carried a snatch block but have never needed it for the purpose of doubling the pulling power. It has been used simply to redirect the winch line pull, but never had it doubled-back. With that said this is a trail truck so most of the time it is needed is when stuck on rocks, roots, or going up a steep hill versus being buried to the frame in mud. So part of the answer to how big of winch you need is how you expect to use it. If you are in the desert and expect to get hung in washouts and rocks a smaller winch will work the vast majority of the time. If you plan on mudbogging and burying the truck in the middle of a swamp you may want a bigger one. Also, while a winch isn't that complicated to use you can optimize the effectiveness of it by using the proper procedures. The less line on the spool the more power it has (think gearing and tire size). A winch listed as having 12,000 lb. pull is the maximum pull it can generate under certain circumstances and not what it always generates. You need to be on the very last layer of cable to generate the maximum pull. The more layers of cable on the drum the less pulling power it has. For example, if the winch has 100' of cable on it you probably need to have at least 90' of that cable spooled out to get the maximum pull. The angle you have the cable attached to also makes a big difference (includes side to side angle and up and down angle). I've seen a lot of people call their winch junk because they spool 10' of cable out and have it hooked at a 45 degree angle to the side and hooked down low (i.e. winch is pulling the truck down against the ruts and basically wedging it down).
Now with all that said, I would probably recommend at least a 12,000 lb. for size of truck.