While I agree that bigger is generally better, it might be nice to know how you plan on using it. Budget, typical use, loading-unloading easily or capacity more important, etc.
You can get by with smaller tires if your sticking close to home, or if loading and unloading as easily as possible is super important. If you're on rough roads, shitty boat ramps, or simply tow it a ton, bigger is definitely better. If your driving up the Clackamas and putting in at a huge paved ramp, it really doesn't matter... One other thought, the bigger the tire the higher your boat will be... bigger tires generally indicate bigger suspension, more material, heavier duty (and curb weight) so balance these issues too. Just so I don't get brow beaten by others on the forum, I'm not saying bigger wheels require heavier duty trailers, it just typically works that when you get one, you'll get the other.
Yesterday's gone on down the river and you can't get it back. - Agustus McCrae