I'm not sure I understand your question, but there is a predictable function from request size to allocation size (for the smaller request sizes up to about 8 kB anyway), and TBB then treats the request as a request for that allocation size, i.e., it forgets the exact number of bytes originally requested, because it would be too much overhead to remember that.
(Added) That's for scalable_malloc, anyway.
(Corrected 2009-07-24) scalable_malloc, not scalable_alloc.
(Amended 2009-07-24) See modifications.
We have added scalable_msize just recently. As Raf corectly said, it does exactly what you want. In the last developer update, scalable_msize is ready to use on Windows; for Linux and Mac, one have to add an entry into .def files for these platforms.Seems we simply overlooked that it is useful on all platforms (and not only as the replacement for MS-specific _msize).