Full Error Message:

Size value is missing

No size value was found for these products. Please ensure that the Product Size field has a valid field mapping in Setup > Describe Categories and that the mapped field contains a valid size value in your cart.

SKU, Missing Attributes size. SKU  doesn’t match any ASINs. Make sure that all standard product ids (such as UPC, ISBN, EAN, or JAN codes) are correct. To create a new ASIN, include the following attributes: size. Feed ID: 0. For more troubleshooting help, see http://sellercentral.amazon.co.uk/gp/errorcode/200692370

Problem:

Amazon requires a valid size value for the product to be listed.

Solution:

Make sure the product has a size attribute that contains a valid size value.

If the size attribute with a valid size value is present then check the size field mapping is in place.