This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
There is a reference to the map:size() function in the streamability rules, but no such function is defined. (My preference is to add the function. Using count(keys($map)) is too cumbersome.)
In fact the absence of this function appears to be an editorial error. It's present in the function catalog, but not incorporated into the final spec because of an incorrect link.
The WG agreed today to reinstate this function. Spec updated.