I have a 16’ with a pair of 7200lb axles. It looks big until the tractor is on it. With the loader and anything on the 3 point, it barely fits. I’d say 20’ is a minimum. Attached a few pics on the 16’. I think you can see why I think it’s a bit small lengthwise.
From a weight standpoint, with box scrape, grapple, loaded R4’s, loader, etc. the tractor is running around 6,000lb. The trailer weighs something. A pair of 3200lb axles ain’t going to cut it. Make sure the payload capacity (gross weight less trailer weight) is at least 7,000lb, preferably a bit more, if you have loaded tires or wheel weights. That’s going to put you with a 10K trailer as a minimum.
Will a F-150 pull it safely? Some will, some won’t. Depends on the drivetrain and suspension on your specific truck. My Tundra is rated for 9,999lb. It pulls the tractor quite nicely. Some F-150’s are rated well over 10K; some are rated 5K. Check the specs on your truck and don’t exceed them.
You obviously need good trailer brakes. Both axles. Being able to go is good. Being able to stop is necessary for survival.
Also, at the risk of insulting your intelligence which is not my intent, make sure you always have positive tongue weight. Around 10% of load is allegedly ideal. You can measure hitch height before loading and make sure it drops a couple inches after loading or get a hitch ball carrier for the truck that has a scale built in. Too much tongue weight, the front of your truck will be floating and hard to control. To little you’ll be getting the trailer sway of death.
Like you, I rarely use my trailer. So why have one? They can be pretty expensive to purchase, it takes up quite a bit of space, is another thing to maintain, if you haven’t towed similar stuff there’s a learning curve and possibly some rig setup to be done, I have to pay taxes on it every year. But I can haul 7,000lb of railroad ties when I need them for a retaining wall, or haul the tractor to church for a work day, or take it to the dealer to have remotes added. I also pull a 9,000lb camper with my truck so the brakes, slightly stiffer suspension, stiffer tires was already there. I have about $750 in the trailer for parts to get it roadworthy. It’s almost 40 and languished in my Dad’s shed most of that time. He gave it to me in exchange for my fixing it and letting him use it whenever he wants. So I have very little money in it. (Some of the pics are pre-rehab which is why it looks more haggard in some pics than others.) If I had to spend $4,000 to $7500 for a trailer, I might be more amenable to paying the dealer to haul it on the rare occasions they need to see it. Of course if you just want a trailer and have the money have at it. Just some stuff to think about…