If you're speaking to people who should know math, use the word 'cardinality.' http://en.wikipedia.org/wiki/Cardinality
"The cardinality of the set A is n."

Or you can say, "The number of elements in A set is n" or very loosely "the size of A is n". It depends on the audience, but probably just saying "the number of elements" would be clear and correct to anyone.