What does 문수 (文數) literally mean?

 ANSWER: Letters (文) [and] Numbers (數)

My Korean-English dictionary defines the Sino-Korean word 문수 (文數) as "shoe size," but it literally means "letters (文) [and] numbers (數)." So, why does "letters and numbers" mean "shoe size"?

Because letters and numbers have traditionally been used to define shoe size: Numbers are used for the length, and letters for the width. 

But that raises the question: If 문수 (文數) is used to mean "shoe size," what is the Sino-Korean word for "bra size"?

In China, the Chinese word for "bra size" is 罩杯 (조배), which can literally translate as "cover (罩) cups (杯)." The Chinese character 罩 (조) is listed under the radical 网 or 罒 (망), which means "net."

From Dong-A's Prime Korean-English Dictionary