Scala - What is the difference between size and length of a Seq?

Nothing. In the Seq doc, at the size method it is clearly stated: "The size of this sequence, equivalent to length.".

size is defined in GenTraversableOnce, whereas length is defined in GenSeqLike, so length only exists for Seqs, whereas size exists for all Traversables. For Seqs, however, as was already pointed out, size simply delegates to length (which probably means that, after inlining, you will get identical bytecode).

In a Seq they are the same, as others have mentioned. However, for information, this is what IntelliJ warns on a scala.Array:

Replace .size with .length on arrays and strings

Inspection info: This inspection reports array.size and string.size calls. While such calls are legitimate, they require an additional implicit conversion to SeqLike to be made. A common use case would be calling length on arrays and strings may provide significant advantages.