filesize -package:http-conduit -package:btrfs

Size of the file in bytes. If this file is a symbolic link the size is the length of the pathname it contains.
Size of the file in bytes. Does not follow symlinks
File size.
File size
Get size of a file in the archive.
File size in bytes
File size in bytes. Read-only attribute represented as a Natural.
-s
Get size of a file in the archive (by index).
File size in bytes
For some files we might not know the size beforehand, but we might be able to provide an upper bound (timestamp, root info)
For most files we download we know the exact size beforehand (because this information comes from the snapshot or delegated info)