If you're looking into a new hoop, I recommend this seller
. The hoops are adjustable, so you can play with it and see what works!
My Twilight Sparkle gala dress uses the adjustable 3-hoop skirt, and I don't believe it's all the way out either. My friend in the Applejack had the same hoop skirt I did, and hers was nearly all the way in, but it could still go smaller, if that helps you get a size idea.
You're a gorgeous Celestia already, I'm sure this addition will make it even better! Please do share photos when you're done!