I'd go with a 6.5 or 7x14 or 7x16 tandem axle. You can look at GVWR to see what the trailer is rated for, subtract the weight of the trailer (1000-1500 lbs) and you'll know how much payload it can handle. You might find a single axle with a 5000lb axle, but more common are the 3k and 3500 lb...