You can select files in a view window by hovering over the view with your mouse. The file under the mouse cursor is selected and marked by a highlighted rectangle; the folders in which the file resides are similarly marked. The name and size of the selected file are shown underneath the view.
You can move the selection from the file to one of the folders in which it resides by changing focus. This way, you can for example quickly find out the size of specific folders. You can change the focus using:
When you change the focus, it will remain at that level in the file hierarchy until you explicitly move it up or down. If you move the focus back down again to a file, the selection will again stick to files, irrespective of their level in the file hierarchy, as it did originally.
Clicking inside the view locks the selection so that it does not change when you move the mouse. Click once more, anywhere in the view, to unlock the selection.
When the selection is locked you can change the folder that is shown in the view by zooming. You can zoom in and zoom out using:
This changes the folder that is shown in the view. By zooming in, you descend into the first subfolder on the path to the selected file. You can continue zooming in, until you've reached the file itself. Zooming out will take you back up again, one level at the time.
When the selection is locked you may also use the toolbar to apply the following actions to the selected file or folder:
Open opens the selected file using the default application that is associated with it in Finder.
Quick Look previews the selected file in the Quick Look panel.
Reveal shows the selected file or folder in Finder. There you can examine it, view more detailed info, rename it, move it, etc.
Delete moves the selected file or folder to Trash. The view will be updated to show the freed space. By default, the control is disabled to reduce the chance of accidental file deletion. Deletion can be enabled from the Preferences.
Beware: Deleting folders will move the entire folder to the Trash, including any files in the folder that are not shown in the view. A file may not be shown because it was masked or filtered, it could not be scanned because of insufficent permissions, or it appeared in the folder after the scan was made. In short, you should be very careful when deleting folders.