On peut pas, sans hacker la table probablement. Ya bien HeapSize, mais c'est de l'approximatif :

HeapSize returns the number of bytes allocated for the heap block referenced by Handle. Due to word alignment and minimum block size, this may not be the amount it was allocated with
. Also note that because of locked blocks, it is possible (rare) that a heap block will actually be bumped up a few words by the HeapCompress routine. So never assume that the value returned by HeapSize is the true number of bytes used by the data stored in the heap block.