Not sure what the diameter is exactly, because I don't know! LOL I use the smallest screwdriver in my screwdriver set. About 10 wraps each gets me to my sweet spot of .5 ohms. They are wicked with organic cotton. Through the coils. As far as the bed setup for wicking, if it works for you, that is great. But, like you stated, the airflow hole in the center makes that difficult. I am guessing there is a millimeter or so in between the posts and the coils. And, if I had to guess, The coils are somewhere around 1-1.5 millimeters in diameter. I also use 28 gauge kanthal wire for the coils. I find the 28 gauge to be more durable then the smaller sized wire(like 30 or 32) most people use for nanos.