Steps to Add Different Sizes for a Product in Magneto 2.X
1. Log in to your Magento Admin.
2. From the left toolbar, click “Store”. A navigation panel will open to the right, and select “Product.”
3. In the new page, under Attribute Code, search for Size and click it.
4. In Attribute Information, select the Properties Tab. Scroll down to the bottom and click “Add Option.”
5. A new layer will be created at the bottom of the attributes list.
6. Enter a size value. The other box is optional.
7. Click “Save Attribute.”
Your size has been added successfully!