I'm not making them, but I'd recommend between 3/16" and 1/4". Anything smaller will probably be too flimsy, and anything much bigger will just seem bizarrely bulky! If you're using eye hooks to screw into the top of the dowel to tie the ribbon to, I'd strongly suggest going with the 1/4" for ease of screwing them in.
I think 14" would be too long - I'd honestly go with 10-12. If you want to check it out, just grab a ruler and try waving it around; it's quite long when you do that!
If you're worried about how far out the ribbons go, I'd just make the ribbons longer.
Re: ribbon wands
If you're worried about how far out the ribbons go, I'd just make the ribbons longer.