You cannot get to see two, or more, 'full' Windows on any screen if you are looking at more than one, but is this important ? For example, an Excel spreadsheet page is potentially huge. I don't remember the exact figures but I think any one sheet can have something like 63,000 rows ! I routinely work with spreadsheets with 2000+ rows and even full screen, you cannot see all of that at once. You can of course use the 'zoom' function in the PDF viewer to allow you to see a complete page even at half screen, but you may not be able to read it at that magnification.
If it is important to work with two or more windows nearly simultaneously, then an alternative is to set them up at slightly less than full screen - so you can see an edge of each of the others - then click on the edge you can see of the next one you want to work on. This is in fact similar to cascading, but this way you are working at almost full screen in each.
The bigger the screen, the more you can see in each window at one time. I happen to have a 19" screen and that works fine for what I do. I have an image viewer in one half and a spreadsheet in the other. What I am doing is cataloguing photos by re-naming them from the data in the spreadsheet, Since virtually all the photos I am working with are in 'portrait' mode, I can get a full height image into half the screen. I can do exactly the same on a smaller screen, it's just that I get a slightly smaller view of the image and I can't see as much of the spreadsheet.
I can't offer an opinion on the cascading method, I haven't used it in years. The answer is to try both and see which you prefer / works better for you.