Parsing array of an object in a library constructor
This is a C++, not Arduino question and should be asked on Stack Overflow.

In C/C++ the [] must be on variable name, not on type. WPCommand wp[] (it is the same as WPCommand* wp). 

sizeof return size in bytes, not element count. send length as parameter

Dynamic memory allocation in MCU is not a good thing. You can have in WayPointStack

WPCommand _wp[arrSize]