Look at other related articles, particularly those in the journal you submitted the article to. This will give you some idea of the appropriate length and content of a conclusions section.
It is difficult to give you specific advice without knowing more about your topic and results. However, when I write the conclusion to an article I generally try to explain several specific things. First, I summarize the major results of the article. If any of the results resolve significant standing questions in the field, I point this out specifically, and reiterate why these answers are important. Second, I explain what further research needs to be done in this area, with specific emphasis on how the current paper affects our expectations for what will be useful to study in the future. Third, if there is a specific avenue of future research that I intend to follow up, I mention it.