Hi Delphine, when you add a product with variants, users can select only one variant by default. In the order form, users can click on the add to cart button for the product, select a variant and the preferred quantity to place the order. If you would like the users to select more than one variant (size), then login to Neartail > click on the form to open it > Edit page will be displayed > In the Edit page (new editor), click on the product to select it > click on the settings gear icon for this product > Product settings page will be displayed > enable the "Allow users to select multiple variants" option and click Save. Please try this and confirm.