Webgains indeed provides a separate datafeed field for size.
However that field is not used by your Webgains merchants - at least not by the main ones I checked like Bench, Boardwise, Rip101, Shore, Surfdome...
Currently that size field is also not processed by Datafeedr.
I've added a todo to process the size field in addition to colour, gender and fabric fields. If any of these fields is used by the merchant the information will be available as tag.