Distortions mainly happen due to not keeping an aspect ratio. For example, most modern televisions and/or displays go by the 16:9 aspect ratio, where 1280x720 (HD 720p) and 1920x1080 (HD 1080p) are both 16:9. To NOT have distortions, you want your camera to also be a 16:9 ratio, so when its zoomed in via the viewport, it is not distorted and maintains its ratio. Having no set ratio is what causes the distortion, as the camera is stretched to fit the view.
An example would be, having a 384x216 camera, and a 1280x720 viewport. This works because both are 16:9 resolutions, so it does not distort when scaled.
The room size is irrelevant.