Regarding the block size versus apparent size, like Padraig, I think it's okay
to let --size just filter the size, whichever the user happens to choose right
now. One can combine it with apparent size.
I think having special character in --size to denote max and min sizes are
confusing. Why not have separate --max-size and --min-size arguments? This way
you can filter by a range, and it's obvious what the flags mean. It's also more
consistent in style with the --max-depth flag.
If you absolutely want just one arg, what about --size=[minsize]-[maxsize]?
e.g. --size=4K- filters output to entries greater than 4K, --size=-8K filters
output to those lesser than 8K, and --size=4K-8K filters output for those
between 4K and 8K.