Adding Product Options and Attributes (Color, Size, Etc.)
How do I configure attributes (color, size, etc.) for a product?
To add attributes to a product navigate to Products / Product Name / Edit. Then select how many attributes you will need in the Product Options area (Note: a shirt with 3 sizes will have only one attribute; size. However the product will have 3 options).
To modify the pricing to reflect the attributes, you will then want to choose price by SKU on the product edit screen.