After making dozens of spear thrower replicas and darts, I have discovered the atlatl and spears kind of fall into two groups. There are the light weight throwers and spears. For the longest time I used only 6 feet long river cane darts 1/2" in diameter on the big end. My large atlatls worked just fine with these and the public who used them seemed to like them better as well. I made a small and thin replica atlatl from the Western US and it was a rotten thrower. Then one day I was throwing with a fellow primitive he had 4 foot long darts that looked like really long arrows about the same diameter too. I tried them with the smaller atlatls and what do you know. They work real well!
So you may need to take stock in what kind of atlatl you have. If it is a non-bending heavy thrower you likely will want a heavier and longer spear. If you have a thinner or flexible thrower you will want thinner and shorter spears.
I hope this helps and good luck.
