How big a winch do I need for my car dolly?

The biggest winch they had at Harbor Freight (we use HF too for everything), was a 12,000lb winch, that's the one we bought. I'll tell ya there hasn't been a time that it let us down. I'm not sure of the brand, I'll get back and let you know if you want. I believe it was somewhere between $300 - $400, but it's awesome. You can pull a full size truck out of the woods with flat tires, stuck in the mud, over tree stumps, 25-30 feet away with that thing... LOL I'm speaking from experience, done it. Get a bad ass winch you will not regret it.