I don't have a Mossberg, but there really isn't a perfect match to a specific model. I use a Rem 870 Super Mag and I'm shooting Rem Nitro Turkey 3" #6. That's just what gives the best pattern with an Undertaker choke. But, it is very specific to the gun and what works best for you. The only way to know for sure is to get out and pattern. Otherwise you're just going out there hoping and either missing or just winging the bird.

If you're just looking for advice on sizes to try out though, I'd look at #4 or #6 2oz. Winchester Supreme or Remington. I use 3" as a personal preference.
What kind of sights are you using?