Size constraints and rendering
A size constraint defines the minimum width and/or height of an image.
For example, if the minimum size is 1280x960 pixels, a user may not select an image that is less than 1280 pixels wide or less than 960 pixels tall.
The user may also not crop the image to a size smaller than 1280x960 pixels.
Note that this does not in itself constrain the proportions - an image of 1280x1280 pixels would be perfectly valid.
Size constraint is not the same as rendering size
A size constraint does not necessarily have to match the intended rendering size.
The constraint simply dictates the minimum size of the original image, or cropping thereof.
In other words, if we set the minimum size to 3000x2000 pixels, we can render the image with any size and proportions as long as rendered image isn't larger than the minimum size - as that would result in a blurry image.
We can always render an image in any size as long as it's not larger than the size constraint.
If the design currently dictates that an image should be rendered with a size of 800x600, it might make sense to set a larger size constraint.
If you would, for example, set the minimum size to twice the intended rendering size (1600x1200), you'll have plenty of wiggle room if a design change requires the rendered image to be larger.
The actual rendering size will be determined by your views, not the size constraint.
However, it should be noted that when using HTML helpers, such as PropertyFor, images are rendered with the same size as the size constraint by default if no rendering size is explicitly specified.