I'm not sure what the company I used recommends or the pro/cons of 18 bersus 20 foot car haulers. Perhaps a guy with more experince can chime in.
Here's the link
www.cargomate.com
Mine is Model EH852OTA2. It is a 20' but measure 23'10" inlcuding the steel triangular hitch in front. There is a 21'10" Model which is probabaly referred to as a 18' trailer. The of course, there is always Serpent Express which a very short and designed specifically for the Cobras and are really nice at somewhere near 16 feet long.