feature req: sync by size only

[sf-man]

i saw that this feature was requested before and closed, but i would
like the dev to reconsider this

syncing by size only is a very useful feature for files within which the
data is not important, such as syncing 2 directories of mp3's

because the modified date can be changed when files are written to
certain devices, yet the file itself is the same, FFS can cause allot of
files to be re-written when there is no need to do so