Originally Posted by Lbear
Stronger and longer are always better. Also remember that you lose pulling power as the line spools out so again longer is better. I don't know what kind of fairlead you have now but it's best to use a hawse type designed for synthetic that has smooth edges all around that won't damage your line.
As lovemachine said, the last two layers of line is where the winch is the strongest, so longer isn't always better.
Also, you need to keep in mind that when winching you're not always going to have it spool in perfectly neat, so it may have to bunch up on one side a bit which can cause issues. You can cram more line on the spool than necessary when spooling it up neat, but when you go to use it you may find that the line is too long.
The length suitable for your winch will depend on the model and size of line. On my XD9000, I'm running ~80 feet of 3/8" line from Viking Offroad. If you're unsure of what to get, give them a call and talk with Thor; he's a great guy to work with and very knowledgeable.
If in doubt, I'd rather go shorter on the line on the drum and carry an extension like shown in the link from lovemachine.