I put smaller wheels on the trailer to make it more stable because it would more easily flip with the 20" balloon tires on it, a complaint made by many who bought the trailer. The 16" wheels are exactly the same that are used by Bikes at Work on their heavy duty trailers.
https://www.bikesatwork.com/bike-trailers/features
"The nylon-composite Skyway Tuffwheels® used on our trailers are extra-strong and never require truing. They use standard 16" bicycle tires and tubes, and roll on precision replaceable sealed cartridge ball bearings."

My trailer is not perfectly level, but when I checked it this morning with a level, half of the bubble is between the lines and that's plenty good enough for me. The trailer pulls nicely and just hums along and is very stable (I often pull tall loads). Plus, I would have to pay more than twice as much to get a trailer with a 300 pound weight limit. So I am very happy with this trailer now and it suits my needs nicely.