Yes! This absolutely is possible, what you are trying to do here is add product variants. This allows you to add different options to a product, in your case different size products. Once you create a product variant, you can set a unique price for that variant.
If that was a little confusing, don't worry! There is a shopify manual that can help walk you through the whole process of adding and editing product variants, you can find that link here: http://docs.shopify.com/manual/your-store/6-products/3-variants.
Essentially you will want to create your product, and add a product option (in this case, size). Once you add your product options, you will now have a product variant. Once this is done you can go edit each variant and set a unique price and quantity for each different product variant.
Hopefully this is the solution you are looking for, if you have any other questions just let me know and I will do my best to help you out. Good luck with store!
I know it is a long time since you posted this answer but I have found it while trying to solve my own problem.
We are an Offshore safety and survival training centre so are selling courses not physical items..
Is it possible to sell variants of a course i.e. 2 courses run together, one has additional elements and costs more and is certificated differently but the 2 courses together cannot exceed 16 delegates.
Is it possible for the 2 variants to both reduce the number of available units as they are being booked and not exceed the 16 without manually manipulating the quantities in the variants field?