Dang! I had to throw away the amaranth sprouts. Apparently I wait too long since I'm accustomed to alfalfa. (Those sprouts fills a half liter container on a deciliter of seeds.) At least I now know what to expect, so I'm putting some more quinoa on soak, and this time I'll harvest in time.
There are quite a few instructive movies on sprouts on YouTube. This one are closest to my own method. The size of the sprout varies with the seed. The alfalfa sprout is about five centimeter (two inches), which means that the volume doubbles five times or more. The beans we see here grow three times their volume, which seems to be the case with quinoa and amaranth too.