Here's the easiest way to size: if you want a 6mm needle, buy a 6mm dowel, and if you want a 9mm needle, buy a 9mm dowel. Whatever thickness you want the needle to be, buy a dowel that thick. I don't know what timbers are sold in other countries, but here it would likely be radiata pine, so be careful about using it for thinner needles, anything under 5mm you need to check out the floppiness of the wood.
I like them! I am not used to seeing the tops that are really flat instead of more rounded, I'm not sure whether I prefer the round ones or I am just very surprised to see flat ones. Love the bead addition on top, the wood looks nice,
If you're hoping to sell them, I would suggest having some rounder beads as the main top as well as the flatter ones you've shown, and see if they both sell equally. I would buy some intense but less bright-coloured beads - so maybe include dark forest and sapphire greens, black, etc. as well as the bright cheerful colours? What's that wood you used? Nice colour and grain, is that the natural colour for that wood?