rdfind may do what you want, but you'll need to depend on a cryptographic hash/checksum (md5, sha1 or sha256) in lieu of the filesize.
The hash is a stronger criteria than filesize, but this may or may not be what you want. For example, consider all of the metadata in a music file: If one file listed
Schubert as the composer, and the other potential duplicate listed
Bruckner as the composer and everything else in the file were exactly the same, the file size filter would classify it as a match, but the hash filter would not. The hash filter will use far more resources than file size filter, but that may not be a concern if you filter duplicates only occasionally.
Before you actually run
rdfind, make sure to read
man rdfind thoroughly and use the
-dryrun option until you're confident the results are what you want.
FWIW, this tutorial lists
rdfind, and 3 other utilities for finding duplicate files.
I don't know of a tool that filters on filesize, but if I were creating one from scratch, I think I'd cobble something together using
awk. Let us know if that's what you want - I don't think it's terribly difficult, but there's not much point if
rdfind meets your needs.
duperemoveto save common extents in multiple files only once, with no other downside)