How to add sizes for one of a kind items in a way Facebook can pull it?

Hello!

I assist a shop that sells one of a kind/one-of-one clothing and accessories. Facebook isn't displaying the clothes on FB or Instagram since there's no "size" listed in the data source. They're referring to a number size or small/medium/etc., not width/length of the clothing.

We list the labeled sizes and item specifications in the description, including the exact measurements. Facebook told me that this information isn't where the "size" specifications on their end of the integration pulls from. It is something additional on Squarespace's end that we need to fill out, which goes into the data source, then auto-populates once they receive the data source. When I reached out to Squarespace, they gave information on shipping size specifications, which isn't what we're really looking for.

I'm not sure what we're missing here, but open to any solutions. Most existing forum post seem to suggest adding variants to specify the sizing there, but that's not applicable to us since these are one-of-one items without additional stock or variations.

Ultimately hoping for a solution that creates a clear indication of either the numbered size or word for the size in the data source automatically, which Facebook can then integrate on their end without manual updates when our store's data source auto-fires to them on the cadence it's run at. Also open to implementing one of the data source rules on FB's end, if that's the best way to go about this.

Thanks in advance for any help!

EDIT: I'm thinking an easy way to navigate this would be to put the size in the "tags" section, then add a data source rule on FB's end that matches the "tags" field with the "size" field, but ultimately our shop owner does use tags for other things at the moment. Would like feedback on if this is the cleanest solution, or if anyone has other suggestions.