BLUEDOG, my trailer is 5400 GVWR. The trailer itself weighs 1200. That leaves a capacity for 4200 for the boat. I figure my rig is about 3600. That is just a guess, but I have a margin of error of 600 and I am cool with that.

You also asked about tires. The very best tire is one that has a spare already in place if it goes flat or blows out -- that is one on a tandem trailer. As for specs - get the biggest diameter, widest and highest load rating you can.