The real impact of the "WideViewAspect" parameter is how the picture is stretched on the window.
If you want to clearly see the difference between true and false values, there is a very simple test:
- switch the sim to windowed mode
- modify the shape of the window, in such a way that it becomes very large but very thin. For example, extend it on the whole screen width, but reduce its vertical size to make it just 10 cms high.
- see what happens.
With WideViewAspect set to False, the sim will always keep the same "lhorizontal" view angle. So if you reduce the height of the window, then you will reduce the visibility of the cockpit. And if you extend the witdh of your window, you won't get a larger angle of view, and your cockpit will be even less visible.
With WideViewAspect set to True, it's the contrary. The sim will always keep the same "vertical" view angle, but the lateral one become unlimited. So if you reduce the vertical size of the window, you will still see your panel, it's not going to dissappear below the lower end of the window. And if you extend the width of the window, then your lateral angle of view will get larger ! (thus the "wide" view).
Anybody flying with a large screen should set the WideViewAspect to "True". You will still have the same visibility of your cockpit, but you will gain some additionnal visibility on the sides

That being said, for a given zoom value, the "amount" of cockpit you can see with True or False is different. Most likely, when you switch from a value to another, you'll have to adjust the zoom value to get your cockpit visibility as it was before. However, if you take a look on various forums, you can often spot some topics created by new users who have large screens, and complain about the lack of cockpit visibility even with extremely small zoom values like 0.40x !! Then these guys get told about the WideViewAspect that should be set to "True" when using wide screens, they try this, and when they come back for feedback, they always say that they now get good cockpit visibility even with zooms like 0.70x.
Of course, this is valid ONLY for people who are flying on widescreens (16/9 or 16/10), and for people that use several screens side by side, to create one very wide screen (triplehead etc...) For people with a single regular 4/3 screen, the WideViewAspect should be set to False.