Hello,
There is something that I have been a lo of time in my mind and hand not asked before and that is why there is NO option to work on the whole workspace size if you want to and be able to make a Zoom to fit the available working screen size.
I mean that when using the reduced preview size you can´t in any way use the zoom, because what it does is enlarge the already reduced very low resolution preview screen so you get very blocky pixels that are not really real and not a useful preview of the real image.
As seen in the attached file I have put there is a very wide grey margin in the work space area of the preview window, and there is no way to fill the screen with quality preview other than choose Preview Size > Actual and then zoom out to nearly fit the screen as you can´t fit exactly in actual or reduced preview as seen on the second attached image.
Even then, using Actual preview you can´t make that the image really is adjusted to fit the work space window size and get the most larged size in the space available.
AND MUCH IMPORTANT, if you really need to change to Actual view, the preview will be really MUCH slower and take a lot of time to preview, so that is why the reduced view was created to make faster rendering preview, but the problem is that is a very reduced previe that can´t fill the screen size and many times you can´t see really very good the resulting image filter effect unless you choose Actual size because there is no option for a QUALITY zoom of the reduced preview.