Even with a homemade trailer the trailer needs to be registered, to obtain a registration it would need to have a declared GVW. There is no way to license a homemade trailer (that I know of) without it being registered.
The GVW of the trailer will depend on the axles, 2 3500# axles would be a 7000# GVW, 2 5000# axles yield a 10,000# GVW. Subtract the weight of the trailer to determine the load capacity. A 16' trailer will likely weigh 1500# to 1800#. Tires also determin the weight capacity, check those! One fairly common trick is to use cheap tires, so you may think you have a 10,000# GVW trailer but only have tires that can hold 7000#. Each tire will have a capacity molded near the bead.