You cannot do it. WhatsApp always shrinks the size of the photo. I am not sure what the maximum is, but I think it is around 2,000 pixels wide. Photos up to that size are sent in full, but above that, they are resized.
The idea is that small photos keep the conversation going and your feed does not get bogged down with massive photos, which are mostly only looked at on a small phone screen anyway.
Use a different method of sharing photos. Email or messengers can be used for example. They often suggest shrinking large photos, but there is always an option to send the full unmodified file. Online services like Google Photos or Drive, OneDrive or Dropbox can be used to share files.