"16. Reorganize Large Diagrams into Several Smaller Ones
It is often better to have several diagrams showing various degrees of detail than one complex diagram that shows everything. A good rule of thumb is that a diagram shouldn’t have more than nine symbols on it, based on the 7 ± 2 rule (Miller 1957), because there is a limit on the amount of information that someone can deal with at once. “Wallpaper” diagrams, particularly enterprise data models or enterprise object models, may look interesting but they’re too information-dense to be effective. When you are reorganizing a large diagram into several smaller ones, you may choose to introduce a high-level UML package diagram (Chapter 6)."
which is based on The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information by George A. Miller in 1956
Scott's book is full of useful tips, and some nice links like this one!