I asked questions about trailers from several people when I started buying Kubotas. My first Kubota was a BX, then a B and then a BX plus an L and then back to a B and BX and on from there to BXs, Bs, Fs and even a Kubota RTV.
Anyway, most people recommended a single axle for the BX, which is usually 3500 lbs weight capacity trailer. Some said dual axle only (3500 x 2= 7000 lb weight capacity) but didn't know why. Have a brother in law that used trailers for transporting "stuff" often. Asked him finally and he said dual axle only and of course I asked why since I didn't need the weight capacity. He said a flat tire on a single axle is a bad, bad, bad or can be a bad, bad, bad thing and logically thinking I could see that so dual axle search began. I found 12' and 16' dual axle trailers as the most standard available. I thought I wanted a 14' since it would handle my BX and maybe some implements and would be less length to worry about. The dealer had 12' and 16' but no 14' on hand. I was sticking to the 14' plan and asked price difference and he told me none, he charged the same for 14' or 16' soooo I left with a 16' which I used for years until this year when I got a new 16' Gatormade with tailgate lift assist (tailgate lift assist essential for me) and traded my old 16' to a 12' (yes, dual axle) used trailer with mostly new deck boards.
Over the years the original 16' carried my BXs, Bs and the L but it was probably overloaded with the Grand L figuring in the weight of the trailer itself but it still did it but I wouldn't, probably, do it again today unless I just had to.
The dual axle 16' will haul your BX and implements which helps make your tractor be able to do lots more jobs. A 14' would also do it but most people think of 12' or 16' which makes them more sellable if you decide to get rid of it. The 16' will also handle bigger "stuff" if you decide to move to a bigger tractor one day.
You will now hear/read from many people to get brakes on your trailer and some states require it and some require it if your weight exceeds a certain number. My first one didn't have brakes (my new Gatormade does but I have no brake controller on my truck) and no more than I use it there was no problem and Ky doesn't require them for what I was hauling. Yes, brakes are better and safer if your vehicle has a brake controller, an expensive add on to vehicles. A trailer with brakes and no brake controller on the vehicle is..... well I guess you can figure that out.