Provide settings for how to scale for different aspect ratios.
It really looks bad when scenes get stretched to fit different aspect ratios. There should be some settings for whether to stretch the scene and change the aspect ratio, or to center it, and just render more background to the sides or top and bottom. The Earth scene is a great example of one that looks TERRIBLE at other aspect ratios.

Each scene handles it’s own aspect adjustment so there is no global aspect. One could be added however that would mean changing over 250 scenes. I’ll add it to the todo list at least
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AdminJoakim Dahl (CEO, Plane9) commented
Each scene is handcrafted and can have any number of ways to compute the aspect it think it needs. One scene might not care about aspect at all. Another scene might use width/height another can use part of the projection matrix all for different parts of the scene. Like the background might not use any aspect while the foreground do to better center the object so the whole object would be visible (Like the earth scene does). In that case it looks good with 2 monitors but as you state might look too small with 3+ wide monitors.
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Kai commented
What do you mean by each scene handles its own aspect adjustment? The only method I've seen in any scene is stretching its default aspect ratio to fit the current aspect ratio. Why is that a part of the scene instead of part of the video settings?