From type/Blob

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method bytes(Blob:D: --> Int:D)

Returns the number of bytes used by the elements in the buffer.

say Blob.new([1, 2, 3]).bytes;      # OUTPUT: «3␤» 
say blob16.new([1, 2, 3]).bytes;    # OUTPUT: «6␤» 
say blob64.new([1, 2, 3]).bytes;    # OUTPUT: «24␤»