You will need to explore the play to find support for your reasons why you think it is a blend of romance and reality. In this way, you will assure your 400-500 word limit proposed by your professor. For starters, though, take a look at the story itself. We have a poor girl who works hard but is getting nowhere fast selling flowers. She happens to be very beautiful (would the story have happened just this way if the girl had been homely? This is a valid question to pose in your essay), and in the process of the "experiment" she and Higgins fall in love. This is part of the romance, but the reality is that if Higgins rejects her, she is still in a not-so-good place. She can't return to her prior life since she has now known a better more sophisticated life and she no longer speaks like the people from where she came. She has no financial means to live like she's become accustomed in the Higgins' household. She could always just marry Freddy, but chances are she wouldn't be happy since her heart is with Higgins.
Eliza is much more grounded and common sense than Higgins. She's lived hard and worked hard, and he has lived a life of privilege his whole life. She is a realist hoping for the best, but expecting to be kicked in the teeth.
There is also the part regarding Eliza's father to consider. He is a bum, drunk most of the time, and he tries to get money from Higgins for "taking" his daughter. Later, he shows up all dressed to the nines with a "I'm getting married to a rich chick" story. Not at all realistic...much more romantic, and it also tends to kick Eliza when she's down since she has done the work of self-improvement and is still not likely to get what she wants (Higgins' love) while her father has done none of this to get his happily ever after.
Take these ideas for what they are worth, and look for support and additional reasons to help you prove that the play is a blend of romance and reality. Good Luck!