The only reason that you're seeing a size requirement for your variation listing is because you mapped the field values incorrectly.
Here's what you need to do:
1. Go back to Step 3 of the Listing Builder (Map the Product Fields)
2. Make sure the category-specific size field (see below) is the one that maps to your Size variation value.
3. Remove Size from any other mappings.
4. Hit next to get back to Step 6, and your Parent Size requirement should be gone!
Remember, you can also hold the SHIFT key, select all of your products, and apply Size Type and Style at once if applicable.