How high was the gallows Haman had built for Mordecai?

Answer: 50 cubits

On the advice of his wife and friends, Haman built a gallows fifty cubits high for Mordecai.

“His wife Zeresh and all his friends told him, “Have them build a gallows fifty cubits high, and ask the king in the morning to have Mordecai hanged on it. Then go to the banquet with the king and enjoy yourself.” The advice pleased Haman, and he had the gallows constructed.”— Esther 5:14 (BSB)