For many years, I was cutting on the land of an older friend, and he had 30 acres of mature PA hardwoods... mostly oak with ash and poplar. We'd leave the poplars rot in the woods, grab all the oak we could, and take the ash and others (sassafras) as we had time or space for them.
Anyway, he was getting up in years, so I'd leave all the really friendly sizes for him, and bring home the stupid big'uns for myself. Lots and lots and lots of trees over 36" diameter, in fast most of the dying ash were right around the 40" mark, making me think they must've all started around the same time.
The largest single rounds I ever brought home were from a 60" diameter white oak that was rotten in the center from the ground up to a height of maybe 18 feet. I left the rotten part in the woods, but brought home everything above that. We figured single rounds at Ø49" x 20" must have weighed over 1400 lb. each. It was fun getting them onto the splitter.
I used to take pics of this, but once it became routine and less novel, got lazy about that. Here's an old one of my buddy skidding some of mine out of the woods, he has since passed.
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Here's some even older, with a 36" bar on my saw for scale, and my old 855 struggling to stack some "medium" (36-inch? tractor tires are 33") rounds after bringing them home. I don't move rounds anymore, only whole logs now that I have a tractor that can actually lift them.
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Good winch is a must, but to be honest, I'd buy one that's faster and less strong than what I have. I bought a 7500# job, which is more likely to break the trailer than anything else, if it ever hits a snag and exerts maximum pull. I only bought that model because I got more than half off, due to a clearance sale, and I had plans to "downgrade" it with gearing out of their faster 5000# variant... but never did.
As to the tailgate, you don't want anything that's going to act as an air brake, speaking in sailing or aeronautical terms. If it's solid, you'll likely want it to be removable for driving. However that might also go along with making it easily-replaceable, since it's going to take a beating. One easy option is plywood or aluminum plate, and some carriage bolts with fender washers and wing nuts. Install it when pulling logs, remove and stow it when driving.
Using a scrap of loose plywood has always worked well enough for me, excepting the fact that I sometimes forget to bring one with me. Thankfully, nearly every time that has ever happened, someone else on-site has had something I could use.