'Amethyst' is supposed to be fragrant but it's either very faint or only something that happens on warm days or when the flowers first open. I just bought this one today and haven't really noticed any kind of odor coming from it. I suspect it's a temperature thing though because I was in a greenhouse earlier today that was full of a couple dozen different Witch Hazel varieties, most of which were flowering and labeled fragrant, but I didn't smell anything.
Flies are the most common pollinator of these plants because of their mostly winter blooming habit. On warm days they emerge from leaf litter and safe hollows in trees and seek out other early flowering plants like Skunk Cabbage. Honeybees also take advantage of these plants for the pollen but only on days above 45F which seems to be the temperature threshold for lots of insects in the winter time.
Size wise they range from small shrubs to small trees depending on the cultivar. Part to Full Sun in a wide range of soil types. I believe they like being on the wetter side of things