Eagle - what I said about panoramic display size has nothing to do with still images. It doesn't really matter what size you make stills because they are not equirectangular images being displayed through a viewing window.
You should always keep the aspect ratio the same for stills when sizing down. If you want to change the dimensions of the image then crop it to your needed size. If you change the aspect ratio you will be distorting the images FOV.
This is not the same for panos.
There are two things in play here.
The ratio of your viewer window and the ratio of your monitor The original question was about the "viewer window size"
viewer size = 600 x 300
viewer size = 400 x 300
600 x 300 will result in the image being stretched horizontally at "a default FOV of 90 deg" You could counter this by changing the FOV but the vertical FOV will be effected.
The image is really a "sphere". You are at the center of this sphere when viewing the pano. It is still the same for Cubes. The box is mapped to the sphere.
This means a SQUARE-viewing window will actually show you the "corrected view" (or aspherical view). You will have a 90 deg FOV both horizontally and vertically
When you display at a different aspect ratios such as .70 which is very close to 4:3 width to height (3/4 =.75 or the other way 1.33) you are squeezing the image vertically. The FOV does not change but the aspect ratio does. This makes the image slightly distorted vertically by being compressed. The FOV is tied to the window size no matter what. When you change the window size your FOV will be compressed or stretched accordingly. It will expand to fill the space but will not change in FOV. This applies to any FOV value.
Now to the Monitor issue:
Ciconia - In Windows, only certain screen resolutions are possible. These include 640x480, 800x600, 1024x768, 1152x864, 1280x1024, and 1600x1200.
640 x 480 = .75
800 x 600 = .75
1024 x 768 = .75
1152 x 864 = .75
1280 x 1024 = .80
1600 x 1200 = .75
Now consider the aspect ratio of the Monitor. A 4:3 aspect ratio = .75
Some larger LCD monitors have a 16:9 = .56
A 1:1 aspect ratio would mean a SQUARE monitor.
Monitor size and screen resolution work together to determine the physical size of objects displayed on the screen.
A 400 x 400 pixel image will NOT be square on a 4:3 aspect ratio monitor. If the height is kept constant the width will be 532 or it will actually be shown at 532 x 400 a rectangle. Now consider that we display at 600 x 400. We must do the math again to fig out what the actual size will be. In this case it is 798 x 400. You can see that the square becomes more rectangular the wider the difference between width to height IF the height is kept constant. The actual pixel dimensions remain the same. The image is NOT being resized in pixels. It is being resized with respect to the monitors aspect ratio. I used actual pixel measurements to illustrate what is happening. I AM NOT SAYING THE IMAGE CHANGES IN PIXEL DIMENSION.
This happens because a monitors native resolution is calculated by horizontal lines, counted per inch, vertically. Both CRTs and LCDs are calculated the same way. For a LCD screen the horizontal lines are actually pixels and not diodes like in a CRT. So the native resolution is based on a vertical basis. This means that you can make the image as wide as you want without changing the resolution but when you make it taller the resolution comes into play. This is why we have aspect ratios.
Place 2 dots on paper. You see them just fine. But how close can these 2 dots be to each other before they appear as one? At this point you have exceeded the resolution of the humane eye (this is expressed in terms of "radian per degree"). In computer jargon this point is 2 radians per degree. I means the human eye can see 2 dots in one radian at a determined "degree" or FOV or another way....how close or far you are away from the two dots.
The 2 dots I mention are expressed in monitor resolution by how close each "horizontal line of pixels" are to each other. In order to increase or decrease resolution the "horizontal lines" will be made thicker or narrower. By making them thicker you decrease the "lines per inch" and making them narrower you increase line per inch. Buy the way, 100 lines per inch is the maximum the average person can see the difference between two pixels. I other posts I have referred to angular resolution in a pano. 100 lines per inch equates to two pixels per degree. So starting with a source image with more than 2 pixels per degree will be giving you added resolution when zooming in. The more angular resolution you have in the source image the more detail you will have at a smaller FOV below 90. To cover different monitor resolutions and different vision ability in people it is best to have a source image of twice this or at least 4 pixels per degree.
So now you ask well how the heck do I make something actually Square on a 4:3 aspect ratio monitor.
A 100 x100 pixel image will APPEAR to be 125 pixels wide x 100 pixels tall. The horizontal pixels are being stretched to a rectangular shape while the vertical lines per inch are not.
To make an image APPEAR square we need to start with a pixel dimension that is the inverse of the monitors aspect ratio. For the 100 x 100 pixel image this would be 75 pixels wide x 100 pixels tall. Although the image will be dimensionally square (if you picked up your ruler and measured it on the screen) the appearance of the image itself will be distorted by this amount 25%. You could factor this in so that the image will appear normal when displayed on a 4:3 aspect ratio monitor but your initial image would look distorted vertically.
A square is not that pleasing to look at when considering a photo. An aspect ratio of 4:3 simply makes the image more pleasing in its shape. At smaller sizes at around 400 pixels wide the distortion is not noticeable to the average person. But when you start displaying on very large monitors it will become noticeable. This whole concept is the same thing that happens to a wide screen format movie when resized for television but in reverse. In this case the image is cropped. In our case the image is widened.
Now to wrap up:
We know that the FOV of your pano window is fixed to the size. If you set a FOV of 90 deg and display at 600 x 300 you will have 90 deg of FOV displayed in 600 pixels horizontally and 90 deg of FOV displayed vertically in 300 pixels. The width FOV will be really stretched horizontally compared to the vertical FOV. Now this 600 x 300 display size is displayed on a 4:3 aspect ratio monitor that now stretches the horizontal dimension even farther. This is an extreme example to illustrate. This effect is less at more normal display dimensions like 568 x 400.
If you stick to a .75 display ratio you will be fine. IF you KNOW you will be displaying on a large plasma screen perhaps for a sales demonstration then you should consider this as your images will be even more stretched horizontally. For normal computer monitors .75 should do nicely.