Você verificou a página de manual de rsync
?
Acredito que a opção --size-only
deva fazer o que você precisa:
--size-only
This modifies rsync’s "quick check" algorithm for finding files that need to be transferred, changing it from the default of transferring files with either a changed size or a changed last-modified time to just looking for files that have changed in size. This is useful when starting to use rsync after using another mirroring system which may not preserve timestamps exactly.