What is the proper size cap or stopper for this bottle?

A 19.5mm-19.6mm cork would work best for a liquor bottle with a 18.5mm opening. For a full list of our in-stock corks that fit, please click here. For more information about packaging your premium spirits, check out our latest blog post. If you have any additional questions, please reach out to our Customer Support team at online@tricorbraun.com