As per this page - http://wwwx.cs.unc.edu/~jsterrel/articles/tcpslice-largefiles.php tcpslice doesn't support a file size bigger than 2GB. Initially, the solution is pretty simple, I've tested it out and it works... add "-D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE" to the Makefile before compiling. But the Makefile is generated dynamically by ./configure - and altering that is a little beyond me :-(
This looks like another one of those upstream packages that Debian has continued to support. They fixed the bug in 2006, and their current version (albeit 2 years old) is significantly more current than that in portage. http://packages.debian.org/changelogs/pool/main/t/tcpslice/tcpslice_1.2a3-2.1/changelog
I'm honestly unable to find which package this refers to so I have to assume it was removed.
That would be: http://packages.gentoo.org/package/net-analyzer/tcpslice Regrettable it's closed, that's a useful tool.