Our plugin uses the WooCommerce Product’s long or short description as Google’s description. You may force either one with the “rule description()” advanced command.
Example: To force the long description using the first 2000 characters,
rule description(long, maximum_length=2000)
See also: Mapping Attributes
As a CRO trick for product feed marketing, you should list the key sales points of your products on the description part.
If that information is in the post content of your WooCommerce, you should use is as your product description while listing products on Google Shopping.