Assuming each screen is 25m of height. Fred is about 6.58% of the height of the screen, Pauline ca. 8.68%, Donkey Kong ca. 12.4%. Based on these ratios their approximate heights in meters are
Fred - 1.64 m
Pauline - 2.17 m (Shaquille O'Neal is 2.16 m)
Donkey Kong - 3.1 m
I was also interested how high Fred has climbed when he reaches the kill screen. Each L5+ level is 150 m and levels 1-4 are incrementally higher by 25 m starting with level 1 at 50 m. That adds up to a total (ignoring the extra few meters that can be achieved at level 22):
Level 22 - 2.9 km (tallest building in the world is 830 m)
I hope someone enjoys these truths