How do you decide an appropriate span of management?

It all depends upon the task being undertaken, the amount of control or supervision required, and the experience of the management. Monitoring a routine or business as usual project, then the span of control can range between 100 (cleaning task for a stadium) down to 8 (sales people) fairly easily. When there is a degree of specialism or monitoring required traditional numbers range from 8 to 4, with about 5.7 being the optimal across business in general (Harvard Business School Number). This number is coming down in recent years - it was thought the number should be a lot higher. When there is high creativity or innovation, or where there is a lot of involvement required and you don't want sub groups automatically being created, then the number can be down as low as 3. The more experienced managers can often handle more people, and there is often a drive by businesses to increase the span of control to drive costs down (less managers). The important thing to remember, there is no optimal value that is constant, it depends upon the nature of your work, your experience of your people and if it is working for you. Don't follow other examples, because you may not know what they do and copying them will not help you. Find out what works for you in different departments and by different managers, and be flexible. Applying an arbitrary number is dangerous. The worst thing to do is giving all your managers wide spans of control, so they spend all their time looking after their subordinates and neglecting their strategy work, seeking out the new opportunities/new business and having time to think to make things better for your company.