For merchants who sell products such as clothes or shoes, size matters. It's necessary to have all sizes (variants) included to allow the customers to select the product of their own size.

Take a dress seller as an example, given the 1 variant limit on the Upsell Product now, the workaround for him is to manually select Size 4 of some dresses as Trigger Products, then set up Size 4 of other dresses as variants under the same upsell campaign. But he has 233 dresses in his shop, and most of them have variants from size 4-12.