Regions don't necessarily track this information, though I suppose they could. You could write a function to calculate the size, using a combination of CacheSPI.peek() (2.0) or TreeCache.peek() ... Recursively iterate over children of the Nodes returned by peek().
What I mean by "not necessarily" is that the eviction algorithms do track size. So, your region would have to have some sort of eviction policy. In which case, you can get at the size by doing a