If you feel that a longer trailer might be useful for other things, a 16' should serve the purpose. Just make sure you have the heaviest one in front of the trailer. That will likely be the Spyder at around 1100 pounds. Always keep in mind, trailer that is loaded heavier in the back (behind the load center of the axles)
will begin to sway at hiway speeds. That sway can get out of control very quickly causing total loss of control and very likely destroy trailer, load and tow vehicle.
If you want to save length, find one or have some custom work done so you can load the bike from the side and have it ride that way. It may require a bit of angle to fit across the width of the trailer but not much; that will depend on the interior width of the trailer and the length of the bike. That configuration should only require about a 12' trailer.
I am partial to hooks or loops welded to the frame around the perimeter of the trailer for tiedowns, but a lot of trailers come with hinged rings mounted on the floor boards. If you go with those types of tiedown points take a look under the trailer to see if they are sufficiently anchored with bolts all the way through, nuts and large washers underneath. The ones anchored only with wood thread lag bolts can pull loose. That is more common on trailers as they age and the wood gets weak, but is something to be aware of in the event you purchase a used trailer.
For the bike DO NOT tie it down on the side stand. That will cause damage to the side stand and any associate safety switches. Tie it down the suspension will be in play as you go down the road. Don't use anything but ratchet straps that are plenty heavy duty to handle the load you are putting under. There are two schools of thought on where to attach straps on a bike. One is to tie it down, so the straps compress the suspension slightly but not all the way down. This allows the bike to ride in similar conditions to having a rider on it. Using that method does require making sure both ends of the straps are secured to prevent them for coming loose in the event any heavy bouncing is encountered. The other method is to tie the front down just above the fork brace so there is no compression of the front suspension and over the rear wheel with specially designed straps for that purpose. With that the bike rides on its suspension like that is no load on it.
For tying down the Spyder, I'll have to defer to someone who has done that as I have never done it--yet.
This probably goes without saying, but make sure your tow vehicle has sufficient towing capabilities to handle that much of a load. I don't know what the HD Ultra bagger weighs but guessing in excess of 800 pounds. That puts your load at around 1900 pounds plus a trailer of that length with a heavy enough frame to carry that much is going to be at least another 2000 to 2500 pounds. The tow rating of your vehicle should be on the driver's side door jamb. Following those guidelines is critical for safety reason and to be legal in a lot of states; maybe all of them. Make sure what-ever trailer you get has good tires. Trailer manufacturers tend to put the cheapest tires they can get much like BRP does with the Spyders.
that's all I got
good luck