To answer the question in the title: Use len() to get the size of a set.

Semantically that is a bit weird, as a set is an unordered collection, and the length analogy kind of breaks down (we are actually measuring cardinality), but in the spirit of Python using len to get the size of all kinds of collections is easy to remember and it is quite obvious what the statement means.

I think you're missing a step in your example. What you are attempting works for me:

>>> values = [ 2, 3, 5, 6, 6, 6, 6, 6, 6, 6 ]
>>> len(values)
10
>>> set(values)
set([2, 3, 5, 6])
>>> list(set(values))
[2, 3, 5, 6]
>>> len(list(set(values)))
4

In your example, you are printing len(values) twice without ever changing the contents of values, so naturally it's going to print the same thing again.

You don't and do not need to.

Python lists grow and shrink dynamically as needed to fit their contents. Sets are implemented as a hash table, and like Python dictionaries grow and shrink dynamically as needed to fit their contents.

Perhaps you were looking for collections.deque (which takes a maxlen parameter) or something using a heapq (using heapq.heappushpop() when you have reached the maximum) instead?