Amazon has certain requirements for how products can be listed on their site. If we do not have the right format for the size of a product (for example we say "L" instead of "Large"), Amazon could suppress the product so it won't be available on Amazon. Below is a list of Amazon's generally accepted sizes*.

When you open the "Edit" field of a SKU, you can edit the size under "Channel Attributes"

It is important to note that the sizes listed above work for most Item Types. There are other item types that have different requirements on how their attributes are formatted. For more information, please reference this guide on Amazon Item Types and download the excel file found on that page.