How can I use nftw(...) and define the block size for the stat function

Question

Hi, I want to use nftw() to traverse my file system directory structure and sum up how many space each directory is using. I don't want to use du because it make fork call. I used nftw (with the flag - FTW_PHYS), nftw calls stat but i noticed that the default block size is 512bytes , while my file system use a different block size.

any suggestions?

thanks!