S4 methods for length which return the number of elements of
  objects defined in the package arules.
# S4 method for rules
length(x)
# S4 method for itemsets
length(x)
# S4 method for tidLists
length(x)
# S4 method for itemMatrix
length(x)
An integer scalar giving the “length” of x.
For '>itemMatrix and '>transactions the
  length is defined as the number of rows (transactions) in the binary
  incidence matrix.
For sets of associations ('>rules,
  '>itemsets and '>associations in
  general) the length is defined as the number of elements in the set (i.e.,
  the number of rules or itemsets).
For '>tidLists the length is the number of lists (one per
  item or itemset) in the object.