This phrase, "My cup size is Stanley," is likely a playful or ironic statement, not a literal description of bra size. Stanley is a well-known brand of thermoses and other drinkware, known for their durability and large capacity. Therefore, the phrase suggests the speaker possesses a large capacity or a significant amount of something, humorously comparing it to the size of a Stanley cup. There's no standard measurement of "cup size" related to Stanley products.
To understand the intended meaning, more context is needed. Where did you encounter this phrase? What was the surrounding conversation? Without further information, it's impossible to provide a definitive interpretation beyond this humorous, metaphorical understanding.
