X'Width is an Ada attribute where X is any discrete type. This attribute represents the number of characters it takes to represent the "longest" type value using the X'Image attribute.
type My_Enum is (Big, Bigger, Biggest);
pragma Assert (My_Enum'Image (Biggest) = "BIGGEST" );
pragma Assert (My_Enum'Width = 7);
-- Width of "BIGGEST" is seven
-- Width of "BIG" and "BIGGER" are irrelevant.