Once the sizes option is selected, you can let the browser know the size of this image in relation to the site layout, using the <em>Sizes</em> field. For a hero image that always fills the entire screen, you could simply enter 100vw, which means 100% of the viewport width. For an image that fills 90% of the screen for small viewports, but only fills 40% of the screen when the viewport is larger than 40em (typically 640px), you could enter "(min-width: 40em) 40vw, 90vw" in the Sizes field. The last item in the comma-separated list is the smallest viewport size: other items in the comma-separated list should have a media condition paired with an image width. <em>Media conditions</em> are similar to a media query, often a min-width paired with a viewport width using em or px units: e.g. (min-width: 640px) or (min-width: 40em). This is paired with the <em>image width</em> at that viewport size using px, em or vw units. The vw unit is viewport width and is used instead of a percentage because the percentage always refers to the width of the entire viewport.