In an ERD (as in, rectangles for entity types, diamonds for relationship types, ovals for attributes, etc.) how would one specify that an attribute is, say, a 5-letter word?
That sounds like Chen notation. I don't think Chen has a standard representation for attribute domains.
If you're drawing the diagrams free-hand (or using a generic diagramming program) then just write it in there. If your using specific ERD software go with what it supports.