How to get the length of a wchar_t array?

Question

C++ / C#

I am reading an image into a wchar_t buffer with a given length and then returning a pointer to that array.
After I need to determine the length of such an array, the problem is that wcslen gives out 5 characters, although there are clearly more than five characters)
This is how the file looks like in a hex editor.

How to solve such a question?