From the latest document that was released with v7.0.2, the verbiage now provides a bit more clarity:  [URL]  [...] Use the largest tape record size available for your version of OneFS. The largest tape record size for OneFS versions 6.5.5 and later is 256 KB. The largest tape record size for versions of OneFS earlier than 6.5.5 is 128 KB. [...]
Page 7 of “docu45326_Isilon-data-protection,-recovery,-and-backup-best-practices-guide.pdf” from October 2012  OneFS NDMP supports a maximum tape block size of 128 kilobytes (KB).    Page 8 of the same document    OneFS NDMP supports a maximum tape block size of 256 kilobytes (KB).   Anyone know which is accurate?
NDMP maximum tape block size