What if I had to write seventy to a hundred words? How many words would that be? I would start by writing down the most important stuff I want to say. Then I would count. If I have too many words, I will have to remove some of the words that I had put on paper. That is difficult. Try to look for long sentences. Can you make them shorter?
Now I have written seventy words. But, what if I wanted a hundred? Try to look for details that you can add. This makes your story more interesting and longer.