To build Criterion::Complex instance. Mongoid add some help method to Symbol. include 'size'.
But 'size' is buildin method. It not work and not a good idea.

I think we must change to another way. We cannot use 'length' because it's same as 'size'.
But like uglily '_size'? Or 'count'(which Symbol not used)?
'$size' operator use for calc array's count. So it's the best one IMO.