The adjective “great” underscores not his size but his terrifying frightfulness. “Red” is the colour of blood and so speaks to this dragon’s bloodthirstiness (Revelation 6:4). As God had indicated in Genesis 3:15, this dragon wants to destroy the woman and her offspring.
3 And another sign appeared in heaven: behold, a great red dragon, with seven heads and ten horns, and on his heads seven diadems.