Ok I found the video.
I think std::size() would not work for those arrays.
You can use
int second[sizeof(first) / sizeof(first[0])];
This would assign the same size to the second array. No extra header files are needed.
If it is a different size() implementation, then mention the video so that I can look up for it.
I checked that the implementation is correct. Check compiler version of Code::Blocks. It should be C++17 or higher.
If the version is C++17 and it is still not working, then add <iterator> header.