I think the spare tire would be in the way.
Anyway, I tied the canoe to the top of the tent trailer, once. I realized when I got to the campground that I had to take it off to raise the trailer and I would have to put it atop the tow vehicle to take it to the launch site in any case. I figure it makes more sense and less work to use the tow vehicle to transport those toys.
I agree; carrying the canoe on the tow makes it easier to take it down to the water. Also, a side benefit is (it seems to me) it helps move the wind over the truck going down the road for better mileage.
There are several schools of thought about length. Some RV parks require a RV 25' or larger. Some Forestry sites have a limit of 25' maximum. I have also seen a maximum of 22'.
So, as you can see we chose 25' for the most bang for the buck for our Full Timing needs/goals.
The question is, "How are you going to use your trailer, how often are you going to use it and where do you like to go?" We like to Boondock, and we like to camp in Forestry camps like the one in the photo above.