Tar: avoid archiving of files larger than certain size

Simpler than you think:

$ tar cf small-archive.tar /big/tree --exclude-from <(find /big/tree -size +3M)

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On a semi-related note (relating to your statement that you can't use find) to get a listing of all files (including directories) under a path minus files larger than 3MiB, use:

$ find . -size -3M -o -type d

You could then do:

$ tar cf small-archive.tar --no-recursion --files-from <(find /big/tree -size -3M -o -type d)

But I'd prefer the first one as it's simpler, clearly expresses what you want and will lead to less surprises.