How can I add a size option to my product on the order form?

To add a size option to your products on the order form you need to define the size as an attribute if is not exist in the system.

Also, you have the option to add to your current available attribute your preferred size as well.

And the third option is if you have an available attribute that can work for your other category that needs to have option size, you just need to add the attribute to your selected category.

Here is the link to the detailed information in this regard: Product Attributes