BFG for git LFS supports the option --strip-blobs-bigger-than
Does git-lfs itself support tracking files based on size, or only based on extension? The git-lfs-track man page seems to only support specifying by extension, so I'm guessing BFS just uses that option to automatically determine a file extension list?
No, git-lfs doesn't support tracking by size, partly due to the way the filters work. See https://github.com/git-lfs/git-lfs/issues/282 .
You are better off to pre analyse and decide which extensions you need to externalize to LFS.