Should be about 5 inches long based on the given length ratio in Origami Masters Bugs , the book it was published in.

If they indeed used Alios Kraft, it's a French product and is machine-made. But I can't find Alios Kraft lighter than 30gsm

There are specialty origami papers (or more commonly, specialty japanese or other asian printmaking papers adopted by origami artists) that are much thinner and tougher than commercially available ones. I do not know where they found 15gsm Kraft since the most common manufacturer used by origami artists, Gascogne Alios Kraft, only makes papers down to 30~ gsm. The lightest paper I've found online was 5~ gsm kozo (a japanese tissue paper) and the lightest I own is 12~ gsm kozo, but I haven't folded it yet. Two well known Western specialty origami paper manufacturers are Paper Circle which makes O-gami, and Origamido. For both, their lightest commercial papers are around 20 gsm but I think they'd try to make lighter ones for custom orders. both papers are incredibly durable and feel almost miraculous to fold if you're used to just crappy 6 inch kami/ "origami paper".

Note that density (weight/area) does not perfectly correlate with thickness but it's usually used as shorthand.