If the size field for the product is populated in the ERP system ( ODSUK_Size for SAP B1), this will automatically create the size on ODSNet.
The size will also automatically create a search filter.
It is common to group products of different sizes and display only one entry on the listing pages.
E.g. a bolt, or a sock, looks the same regardless of size. It makes sense to group all size variations ( see product variations) and only display one image on the listing pages. When a user clicks on the product on the listing page, it will open a product page with all size variations linked on one page.
For apparel, there is a specific product page layout, which enabled the users to enter quantity per size, and then click "Add to basket".
See also: Product variations, Search filters