If you don’t know how to fit the whole picture on instagram using the Full Size feature then you must follow these steps:-
1. The first step is open Instagram in your device and log in to your account.
2. Then tap on the + icon which is located at the centre of the screen at the bottom.
3. Now select the photo that you want to post.
4. Then tap on the Expand option in the lower left corner.
5. Now pinch the screen using your two fingers and Zoom out until the photo becomes full size.
6. Then finally upload the photo.