Welcome, the Malone LowMax trailer is described at
https://maloneautoracks.com/XtraLight-LowMax.html as being 11 feet, three inches long overall. This should be fine with a single 18 foot long canoe that is centered over the tongue. Some experimentation is always a good plan as Glenn suggested. A bigger trailer has been listed at the link below if your testing is not successful. Keeping the load far enough forward to maintain a positive tongue weight is also important. (Don't ask how I learned this to be true.)
My old canoe trailer died a few weeks ago so I recently purchased a Malone MegaSport trailer. I occasionally need to haul multiple 18 foot long wooden canoes and other heavy stuff so this should work well. Good luck,
Benson
1997 Family Four. by MO Trailers. 14 inch wheels , spare tire, electric all works, Marine plywood box that still seems intact. Open top. The frame for the box is all steel. Similar to this but the box sides are wood in a metal frame and my trailer carries four craft. I will...
www.canoetripping.net