Magento change Custom Option value before adding it to cart

Question

I have setup a custom option for my product in Magento as dropdown i.e.

Size : Small, Medium, Large

On product page I show additional information for each op

Answer 1

Go to Admin -> Catalog -> Attributes -> Manage Attributes. Locate your Attribute from the list. In your case, it's probably size. Click it and go to Manage Labels / Options. From there you can add the additional information to each value. You can change the Label to "Small - Waist 30, Chest 36, Length 42" and repeat for each attribute value that you want to change.