Hi - you can achieve a lot with specific prompting. I use GPT 4 for summarization a lot and I have over time shifted to a prompting strategy that involves giving the model the general instruction for preparing a summary and then appending that by a list of specific principles that it should adopt when creating this summary. In my case that’s over 10 specific principles that address things from style, structure and granularity to other very specific points unique to my context.
Getting your output in the desired shape will involve some trial and error. Review the outputs for characteristics that you like/dislike, then incorporate these findings into your prompt.
But in a nutshell, if length is your concern, then that can definitely be addressed in your prompt.