this was already discussed in the past, and 1 or 2 solutions were suggested.
Problem is: calculating the size of a folder may take forever, best thing is to pre-calculate and store.
But this sets another problem: keeping the information up to date.
And i'm not even considering the additional problems HFS brings: you can set a filter and hide some files, so when you enter the folder you may not find as many bytes as you showed outside.
I don't have a good solution for this, and until that you won't find an embedded solution.