How to get stock levels with my options???

I have been able to set up my products with options and corresponding prices quite easy thanks to Zen Cart. And probably due to the fact that I only have different content options to my product .......
For example:
- shampoo X with content options 200 ml, 500 ml and 1000ml

I am now trying to get these options into my stock as well. My stock level for shampoo X shows e.g. 10. I have no idea however what content is in stock. Is this 10 Shampoo X of 200 ml, 500 ml or 1000ml? The stock for Shampoo X should be devided depending on the options available.

Does anybody know how to add stock to the separate options in stead of just one total per product?