Update size parameter to accept callable

Allow 'size' parameter to be a callable that returns an int.

Author: OneSizeFitsQuorum

fsspec/callbacks.py

@@ -91,8 +91,10 @@ def set_size(self, size):
 
         Parameters
         ----------
-        size: int
+        size: int or callable
         """
+        if callable(size):
+            size = size()
         self.size = size
         self.call()