The executive summary should ideally be one or two pages, and no more than 10% of the whole report's length.
Although they're often treated as an afterthought, executive summaries are very important documents in their own right. They need to be able to act as a gateway into the full document โ or even a replacement for it, for the very busy reader.
Indeed, they are tough to write exactly because they need to be short! They should contain only the most crucial information (and nothing that doesn't also feature in the full report).