I went through some of the same decision process as you. Originally I purchased a
bx2360 without loader as well as a 10' trailer to move it around. The trailer worked well until I added a loader to the bx. At that point I upgraded to a 18' 7K trailer. While plenty of folks use a 7K trailers to move their subcompacts around with no issues I found that the cross member spacing on mine was such that there was considerable board flex when the tractor was loaded and the whole trailer frame had enough flex that towing was a disconcerting experience.
Eventually I purchased a 20' 10K with cross members that were I believe 16" O.C. and have been very happy with the purchase. The trailer is strong enough that I can load anything I can feasibly need on to it. While you certainly don't need 20' of deck, it does allow for some flexibility in loading and balancing. I actually purchased extra length with the intention of mounting a toolbox to the deck giving me 18' of deck space.
My point is that while you likely can get by with less than an 18' deck, the space sure does come in handy in terms of balancing loads and securing loads. To me having that flexibility about load placement can't be understated because it can significantly impact the ride characteristics of your tow vehicle, especially if you are thinking of hauling more than just your tractor. The flip side is that with extra length comes extra weight and lowered hauling capability. My trailer weighs right at 3K so I've likely lost between 500 and 1000lbs of loading capability compared to some trailers. That is likely unacceptable for some folks, but was alright for my circumstances as I was willing to trade hauling capacity for better handling characteristics. Your circumstances and tow vehicle may necessitate a different set up. Good luck! Have fun looking.