If you keep more weight ahead of your trailer axle than behind it, your trailer will not sway, regardless of tongue length. Pay attention to your trailer loading and use a little planning. Tongue length only comes into play when you're backing a trailer with the tow vehicle - a longer tongue is easier to back than a shorter one due to its greater turning arc. This is not an issue with a bike as you won't be doing much backing. Of course, a longer tongue makes a great place to add a cooler (and add more weight ahead of the trailer axle).