https://github.com/vbatts/tar-split/blob/master/checksize.go

vbatts@valse ~/src/vbatts/tar-split (master) $ go build ./checksize.go
vbatts@valse ~/src/vbatts/tar-split (master) $ ./checksize ~/d.tar
inspecting "/home/vbatts/d.tar" (size 1420749k)
 -- number of files: 38718k
 -- size of metadata uncompressed: 43261k
 -- size of gzip compressed metadata: 2251k