tar-anal-blocksize

tar-anal-blocksize is a variable defined in `tar-mode.el'.
Its value is

20

Documentation:
The blocksize of tar files written by Emacs, or nil, meaning don't care.
The blocksize of a tar file is not really the size of the blocks; rather, it is
the number of blocks written with one system call. When tarring to a tape,
this is the size of the *tape* blocks, but when writing to a file, it doesn't
matter much. The only noticeable difference is that if a tar file does not
have a blocksize of 20, tar will tell you that; all this really controls is
how many null padding bytes go on the end of the tar file.

You can customize this variable.