The correct setting to display lightning-layout-items in one column on small devices and in two columns on tablet-size and desktop-size screens is to set size="6" small-device-size="12". The size attribute specifies the relative width of the lightning-layout-item within the lightning-layout. The value can range from 1 to 12, where 12 takes the full width of the parent container. The small-device-size attribute specifies the relative width of the lightning-layout-item on small devices, such as mobile phones. The value can also range from 1 to 12. By setting size="6" small-device-size="12", the developer can make the lightning-layout-item take half of the width of the parent container on tablet-size and desktop-size screens, and the full width of the parent container on small devices . Reference: [lightning-layout-item Component], [lightning-layout Component]