With an open trailer, 16' to 18' is plenty big enough. Take a look in my profile. Every car/truck I have listed has been hauled on my 16' tiltbed. Even my '46 Chevy COE.
20' I have a short roadster that I used to haul on an open trailer, then load the scooter, sometimes drum of fuel, pop up, you name it. You can always pull a trailer with more space than you think you need but you can't add space to a trailer that is to short.
My last one was a 16. Next one will be 20 - 22, thinking about 16 of it as a tilt, maybe even a triple for better tracking. I always bumper pull with my C class. I can afford only one trailer and I haul a lot of the kind of 'stuff' that is much easier with an 'open'.