Here's a pic of my 14 ft. raft on my 12 X 6.5 trailer. Not the best pic but you can get an idea. I transport my raft fully rigged & ready, from Salt Lake City to Idaho a lot. The picture is at Corn Creek put-in for a Main Salmon trip.
I'm a 5'5" female who loads my raft solo. My trip prep goes like this - I inflate the raft and slide it onto the trailer. Then I put the frame on and strap it down. Then I roll the trailer from my side yard into my garage. Next I load in my empty dry boxes, cooler, and put all my accessory stuff like my captains bags, gear net, etc. into place. I can adjust straps with the empty boxes & cooler to make sure everything is in the right position. Next I load the dry boxes & cooler. Then I load all my loose gear into the cargo area of the raft, which it on the front of the trailer. I strap my oars to the frame. Everything is strapped down securely, and I check straps & raft inflation at every gas/bathroom/food stop along the way. I've been doing it this way for years without incident.
There have been a couple trailer threads recently, so you can check those out to see what others do. My trailer started life as an ATV trailer. It has been modified to be a "flat bed" type raft trailer, as well as a cargo hauler with sides, and now a kayak/canoe trailer. I finally have some pictures that show the trailer with the sides on. I just added 2 pvc cross bars to the trailer to morph it into a kayak/canoe trailer.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb