What does this ['s] mean in [double D's]? - She's already got C cups, but now she wants double D's! Is it possesive ['s]? or plural [s]?

It would be plural, more than one. It refers to a woman’s breast size or more specifically the cup size of her bra.
HiNative is a global Q&A platform where you can ask people from all over the world questions about language and culture. We support over 110 languages.

Momo's Work, PDF, Pronoun

How to sound 🇫🇷French when you say “Enjoy (your meal)” → say 'Bon ap, French

Chris's or Chris': The Correct Plural Possessive Form

Beta Theta Pi Style Guide (2023-24) by Beta Theta Pi - Issuu

My Double D's and The Love/Hate Relationship I Have with Them – The Slightly Spiked Latte

The Blue Book of Grammar and Punctuation

punctuation

Do NOT mistake this word when in France 😳!! You may think that 🇫🇷

Numeral Classifiers and the Structure of DP - Linguistics

Grammar

Mark 14 - In Depth Bible Commentaries