The gravel builds up gradually from a 1/2" at the low end up to 6" toward the center. Really it just needs to be
enough to allow excess water to drain and not sit in the soil.
Yes, I have had many herbs such as rosemary and thymes overwinter in spirals where they have not out in open beds.
Shady characters.. try
land cress and fiddle head ferns
I'd recommend about 3' high. I haven't hooked up irrigation but a cool design idea is to run a sprinkler head up the center as your building it, though it'd be a pain to fix. I defer to drought tolerant species - mainly Mediterranean herbs. My book describes a free form method of building a spiral that uses a lot more soil mix than stone which holds moisture well.