This dilemma has come up several times on this forum, so search for many opinions. Tnttt.com has a whole forum with thousands of conversions and will be a wealth of information. A lot will depend on you, where and how you want to travel. If you are under 6' tall a 6' wide trailer gives you more floor plan options but doesn't pull as easily or go down narrow forest roads as easily and usually requires wider mirrors to see around plus being generally heavier than a 5' wide trailer. A single axle trailer is lighter than a dual axle but the tire wear is greater because the load is carried by 2 tires instead of four. A flat tire is not good on any trailer but it can be very damaging on a single axle and is more likely to cause control issues as well as prevent continued progress down the road. If your Four Runner is a V6 fuel mileage will suffer greatly with a 6' wide dual axle heavier trailer. Camper made a wise choice in my opinion by using a V8 Tundra pulling a light weight 6' x 12' standing height trailer. I have found it is easy to overload with all the gear required for full time living and pulling it down rough unpaved remote roads for me meant a dual axle. I started much like you with a topper on a Tacoma then added a 4' x 6' single axle utility trailer. Later I enclosed it to sleep in bad weather when I couldn't use a tent, then enlarged the trailer and added a bathroom which made it 80" x 11' which was way too much for my tow vehicle as the gas mileage dropped by half in head winds. So now the camper portion is mounted on a 6' x 24' flatbed dual axle trailer which allows me to haul a Suzuki Samurai on the back 12', mount plenty of solar and carry all my camping gear and kayaks. I pull it with a 3/4 Ton modified Suburban (10 mpg) and use it as a base camp while using the Samurai (25 mpg} for daily trips. A light weight 5' x 8' standing height trailer would work fairly well with your present vehicle as long as you don't overload it. Otherwise you will end up somewhere between Campers tow vehicle and trailer is and mine. I should mention there are two of us, my wife and I, which does increase the requirements. Good luck and hope you find quickly what works for you without too much expense and discomfort.