ok, its an atlatl to most folks, but when there are giant critters to be found nearby, one has to think big!
My 8 year old hunting partner visited this past weekend, and we had to get up a kit for him. I made the thrower the traditional "elbow to middle finger tip" length , (12 inches), and we tried out a couple of different diameter hardware store dowels for ammo. At 48" for the dart spear, the 1/4" diameter one seemed to work best, going tail down/ point up if the throw was not very hard. Of course an 8 yo doesn't throw easy very often, with most shots going anywhere and everywhere, and some even making a decent arc towards the wooly mammoth milk jug.
pics show the 1/4" dia trout bead and a bit of 1/4" pex used for the spur/nock arrangement. One dart broke into three pieces with a energetic throw by the mammoth hunter, and I had to make a field reassembly with electrical tape, figuring he dart would be flakey as it was quite limber, I was not surprised with its poor flight. I was surprised when I tried it in my 20" thrower. It survived the day being thrown rather hard by adults, and flew quite well from the normal length thrower, although the range was not spectacular, as it weighs less than an ounce.