Does anyone know what the maximum size boat you can put on their 600-lb capacity boat trailer? I have a 16' 70" wide jon boat that I need a trailer for. I cannot for the life of me find any information online. I appreciate all your help. Have a good one.

If that's an aluminum jon, depending on the size motor it would probably work, but if you have more than a 25-30 hp motor, you will probably exceed your 600 pounds very easily if there is much on/in the boat.

No outboard on my jon. Just a TM. Boat is 400lbs with the deck I built for it. My question is: will my 16' jon fit on this trailer?

From past experience of using basic utility trailers, you will have to greatly extend the tongue.  That's easy enough to do just by buying a stick of steel like it, drilling the holes, a spray can of red paint and a little work since there is no welding required on that one.  A whole stick of steel is going to be 20-22 feet long but most places charge almost as much for a drop (a length cut off a whole stick) that it's about as cheap to go ahead and buy the whole stick.  Plus, a lot of places have quit selling drops.  Some welding and fab shops may have a piece about the size you need.

If you don't extend the tongue and get the wheels closer to the rear of the boat, if it does go on it, it may bend the hull with as much sticking out the back as you will have.

One other thing I thought about, if you are not planning on adding bunks, rollers and winch, make sure that distance between the fenders is enough for the bottom to clear them.