You can choose whether or not a product appears in your Wholesale Marketplace (Buyer Store).
First, go to your Products page and open the product.
Next, click on the tab: Sales Channels:
Under the Sales Channels tab you can toggle between Active and Inactive on each product variant on the BC Wholesale Marketplace line:
If the product variant is set to Active, then that product variant (size) will be available in your store for buyer's to purchase.