Not sure if you mean committee reports or scrutiny review reports so will comment on both. In terms of committee reports we have a rule (for all reports, not just scrutiny) that the covering report must be no more than 2 sides of A4, with further documents attached as appendices. Although the appendices can be quite extensive, it does mean that anyone can get the 'story' of an item by just reading two pages. The two page report includes background, financial appraisal (if appropriate), key points and recommendations. I think this works well as a discipline.
On scrutiny review reports we have a standard template which means there is a common format which Members and other officers can get used to. Although we try to keep reports concise, if a report must be lengthier for some reason we will include an exceutive summary which can stand alone if necessary.
You can access examples at www.eastsussex.gov.uk/scrutiny