To lengthen your paragraphs in fiction, try to use more complex sentence structures. Incorporate subordinate clauses. For example, 'Although she was scared, she knew she had to face her fears as she stepped into the old, creaking house, her heart pounding in her chest like a drum.' This not only makes the paragraph longer but also adds more depth to the description. You can also add background information or flashbacks within a paragraph. Say your character is at a party, you could briefly mention a past party experience that relates to how they're feeling at this current party.
You can combine short paragraphs. If you have two or three that are related in theme or action, merge them. For instance, instead of having a paragraph that says 'She entered the room.' and then another that says 'The room was dark.', you could make it 'She entered the room. It was dark, the shadows swallowing the corners like a hungry beast.' Also, expand on the consequences of actions. So if a character makes a choice, write about how that choice immediately affects the situation and the characters around them.