Your rabbit, including ears, takes up roughly 70 vertical pixels out of a total of 1,288 vertical pixels.
80% of 70 is 56 pixels. 56 pixels divided by 1,288 pixels is 1/23 of the image height.
You'd need roughly 23 x 50mm, or 1,150mm, to fill the frame with what 56 vertical pixels is currently showing.
You might as well round it off to 1,200mm, because anyone selling an 1,150mm lens will market it as 1,200mm, anyway.