Having had a few trailers, I'll put in my two cents. I had a 5 X 12' tandem trailer built by Indian Valley trailers for my lawn business. 5' because in my former house I had to put everything in the garage every night and the overall width would fit. It originally had a landscape gate that was destroyed by a large maple tree falling on the trailer during a storm.
Before this I had a single axle. If you want a trailer of any size, I highly recommend the tandem. The equipment rides smoother, you can unhook it easier with a load and the safety factor is worth every penny extra. I picked up mulch one time and after being loaded, the loader guy pointed to my flat tire. Still able to get it home and not be stranded. During projects, I use it to haul topsoil (2 tons), rock , mulch, and everything else. The higher capacity means less trips. After getting the load home, I hook it up to the BX and drive the stuff to where I need it. I've hauled my BX with the tandem and as others have said, it's not long enough for the tractor and an implement and mine can't haul the tractor with the MMM because of the width. I would get at least 16' if i hauled the tractor a lot, but that size would get limited use for me behind the tractor. With a tractor behind me, I would not have a trailer without brakes, especially if you get up to highway speeds.
I guess it depends on the main use for the trailer. If you haul the tractor a lot, I'd go big as you could and tandem with brakes. If the main use is for other things, I'd size accordingly. I use my trailer a lot behind the tractor. I use it to pick up brush and leaves, transport firewood, and as a mobile work platform with the generator and other tools. A truck is nice but the trailer is just real versatile especially when you have a tractor to pull it. Good luck.