What size would the base of this retainer be?

I’ am getting this retainer made for me as i live in the UK and its expensive to import this retainer, so could you please tell me the size of the base plate (the bigger bit on the inside of the mouth) is.
This is for a lip piercing

http://www.bodyartforms.com/productdetails.asp?ProductID=1942

maybe and 8 ga or a 6 ga, its kinda hard to tell

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