In the Fullscreen Preview window you can mark assets by using labels and ratings.
Color labels are set using the square symbol. Clicking on this icon opens a list of available labels. Select the label you want to mark the asset with.
You can also mark assets using star ratings. Use the shortcuts CTRL+ALT+1-5 / ALT + CMD+ 1-5 to set the rating. See “Labeling”, for more about labels; see “Rating” for ratings.
Adjusting Preview Size
The magnify glass icons of the statusbar let you scale how an asset is displayed within each preview area (panel). To zoom in on a preview image, you also use the + (plus) and - (minus) key shortcuts.
Rotating Previews
The rotation icons of the statusbar let you rotate a preview 90° clockwise or counter-clockwise.
CTRL+ALT+1-5 / 
ALT + CMD+ 1-5 to set the rating. See “Labeling”, for more about labels; see “Rating” for ratings.