Automatic price increase or decrease when size is selected

Hi guys,

Please help!
I am building an online shop selling hair. In the shop, I have different prices for different sizes of a particular product. I have created an “attribute” for customers to pick a size, but the problem is, how do I make the product price change and a size is picked.
Example: Let us say that standard size of 8inch hair display is $10. I have attribute that customer can change the size. Let is say for the same type of hair that is 12inches, the price is $14.
Please how will I configure my theme so that when a customer chooses 12inches, the price displayed will automatically change to $14.

Thank you