You set the Item Specific from attribute, but anyway you got error “Listing is missing required item specific(s): “Size”. (This error might be displayed not only for Size, but for other options i.e. Shoe Size, Dress Size, etc )
Usually such situation appears for configurable products, when “Size” is presented in Item Specific and also as an option in variations. There is no value for custom attribute in configurable product as all values are kept in the associated simple products.
So to avoid this error message, the Item Specific “Size” should be set not from Custom Attribute, but as Custom Value. You can put it some generic info (i.e. varies, L-M-XL, etc)