The length is related to your story and additional questions your interviewer may ask.
I recommend using the STAR framework:
- One Sentence summary of the story
- In Situation, you should briefly provide the context, usually in 1 or 2 sentences
- Task usually includes 2 or 3 sentences describing the problem and your objective.
- Then you provide a list of specific actions you took to achieve the goal. It should take 1 or 2 sentences per action (Usually 3-4 actions). Note that the interviewer can stop you any minute and ask for more details.
- The results part should have 1 or 2 sentences describing the outcomes. This part is finalizing your story - make sure it can impress the interviewer and stay in the memory.
It is important that the Interviewers will drill down into your story to understand the dynamics, your reactions and decisions and most of all the rationale behind them. They will follow up with other questions as you tell your story to make sure they understand it in enough detail. I recommend preparing for these additional questions in advance.