If you are using a single hammock and trying to keep the weight down, then you really don’t need a large hammock fly at all. You want at least 12” of length from top to bottom, which in this case is from the ‘head’ strap to the ‘foot’ strap. The lateral length should be at least 8 feet. This is the standard size of the smaller rain flies.
However, there is nothing stopping someone from bringing a larger fly than they need. Having some extra space that is protected from the elements can be incredibly valuable if the weather goes bad in the wilderness. It also allows for the use of a double-wide hammock, which offers up a whole new level of outdoor fun.