On this page I have added Related products.
- where is the select button, to select a product?
- those shirts are in different sizes. How can I display the
Sizeattribute here? So users are able to select their size directly?
If related products type is configurable or have custom option(with required) type then cannot be cart directly.
So you need to customization, and in that case you can use this link Configurable product on list.phtml page