How wide is a 16 foot dual axle trailer?

Varies by make and model. Since you're using English/Imperial measurements, rather than metric, I'll assume you're in the US. The max width allowed without special permit is 102 inches. 96 inches is also a common measurement. Again, there are several different types of trailers, and I'm assuming that you're referring to something like a tag along trailer for hauling heavy equipment, although the question could just as easily be in reference to a boat trailer.