At the time the constructor of your class is called, its size must be already defined, because the memory allocation takes place before. But you try to give it as a parameter to the constructor. This cannot work.

You have at least these alternatives. (Note: This is an invitation to edit this answer and add more, if you know some serious solution.)

1. Use a template for the size

Please be aware that each different size will generate another implementation in machine code.

template<size_t size>
class myLib {
public:
    inline myLib<size>() {}
private:
    bool data[size];
};

2. Allocate the requested memory dynamically

class myLib {
public:
    inline explicit myLib(size_t size) : data(new bool[size]) {}
private:
    bool* data;
};

At the time the constructor of your class is called, its size must be already defined, because the memory allocation takes place before. But you try to give it as a parameter to the constructor. This cannot work.

You have at least these alternatives. (Note: This is an invitation to edit this answer and add more, if you know some serious solution.)

1. Use a template for the size

Please be aware that each different size will generate another implementation in machine code.

template<size_t size>
class myLib {
public:
    inline myLib<size>() {}
private:
    bool data[size];
};

2. Allocate the requested memory dynamically

class myLib {
public:
    inline explicit myLib(size_t size) : data(new bool[size]) {}
    inline ~myLib() {
        delete[] data;
    }
private:
    bool* data;
};

Note: Depending on the version of C++ you're using, you need to consider the Rule of Three (before C++11) or the Rule of Five (beginning with C++11). Since the implementation of the missing methods depends on specific details, it is left as an exercise to the reader.