CNY,
make a trip to the dealer if you need to and measure the exact setup you want to trailer. I have been advised by the dealer that 18' was long enough for my 3130 fel & bh90 setup, "thats what we sell all the time" and they also told me I couldn't have anything hanging over the deck on any end or side. They told me 18" was ok because they didn't have anything longer on the lot. I measured my machine and it's gonna take a 20' to keep it all inside.
Don't buy too short, you will not like it for a long time to come. The extra length only costs a few more bucks and offers so much more capability.
Bottom line.... measure and buy one that fits.
Best of luck shopping,
Martin