I have that trailer with 12-inch wheels. The tires are marked 55 mph max, and I stick to that to reduce wear and tear on the Mazda6. It means I get to TTAC in 20 minutes instead of 15. It’s funny watching traffic on I-25 slow down to eyeball the kart. The bigger tires are preferred from a load-capacity standpoint (not that a kart will tax that trailer at all), plus they spin at lower RPM at a given road speed. I’m operating nowhere near the limit of performance of the Mazda or the trailer… I’ll leave the envelope-pushing for the track.
Note that the fenders on the 12-inch model stick up above the trailer floor, so you’ll either need to dispose of the fenders or build up rails for the kart to sit on. Raising the kart is better from a load-securement standpoint anyway, since your hold-down straps wouldn’t be as parallel to the trailer floor. [EDIT: Now I see the replies just above mine. My side pods run into the trailer’s fenders, but then again my ‘pods are ancient “bread boxes,” and I’m not carrying additional cargo on the front of the trailer, so I need to move the kart further forward to get the weight balance right. YMMV.]
As for the hitch, there’s a wide range of bars with different drops available. Just get the one that puts the trailer floor at the desired angle. If I remember, I’ll measure the hitch height on my trailer tonight. Then you just need to measure the height of your hitch receiver (the square hole that the bar slides into), then account for the height of the ball and there you go.
In the end, since I carry nothing but the kart on the trailer (everything else rides in the station wagon), I probably could’ve gotten away with the smaller Harbor Freight trailer. However, my wife (the woodworker) can haul full sheets of plywood more easily with the bigger one. Maybe someday I’ll build a locking box for the kart to sit on that would carry some gear.
Just my $0.02,