How do I save a list of sizes like S, M and L so I can reuse them on every product?

Save them once as an attribute.

1. Go to Products → Products and click Attributes in the links under the page title.
2. Click Add Attribute, enter a Name such as Size, then click Save.
3. On the Size row, click Items. Type each value in New item (e.g. Red, XL) and click Add item. Click Close when done.

When you add variants to a product, type Size as the option name. Your saved values appear as suggestion chips, such as "+ S" and "+ M". Click them to add them instead of typing.