Its pretty clearly explained in the manual on page 217 (volume 1)
Basically, the preview mode shows the video at its normal size unless it is larger than the monitor in which case it scales it down to fit the size of the monitor.
In Raw mode it always shows the video at its true size with as little processing as possible, even if it is larger than the monitor…in which case you dont see all of the image.
Generally you will want to use the “Digital Cinema Desktop Preview” mode if you have a dual-monitor setup and “Digital Cinema Desktop Preview – Main” on a Powerbook.
Sterling Noren
Owner/Writer/Producer/Editor
WideWorld HD Productions
Seattle, WA