I think the species and width of the sticker is just as important as the length. I like to make them of dry poplar, and usually about one x one inch thick. Place them about 12 inches apart on good lumber, making sure they line up over each other, supported on the bottom as well. Mine were 52 inches wide. My forks were 48 inches long, and I could make shorter, wider stacks, easier to sort through later if necessary. Tall stacks are a pain in the ass, if boards split in the middle. I've heard of green stickers leaching out on drying hardwood stacks, making mold and stains. It's kind of a learn as your go process, do what works best for you in the long run..
My two cents.