Thank you veruy much for this it is really helpful.My cover letter was 935 words - I understand that I may be the exception, not the rule. But I must say that every word I included in my cover letter added to the overall application, which made me progress to the next stage.
I don't think the issue is the length of the cover letter per se,instead it is what you are writing adding value to you standing out as an applicant to graduate recruitment - because every member of that team reads the application extensively.