How to modify Media Library images

A quick look at the media page with Firebug shows that this CSS statement will allow you to change the size of the picture:

.media-frame-content[data-columns="9"] .attachment {
width: 11.11%;

}

Change the ‘width’ value to something different, with the understanding that the change may affect how the page looks on different size screens.

You should put your modified CSS in the styles.css of a Child Theme, since any updates of WP or the theme (which might have their own media page) would cancel out your changes.