Dimension’s primary output is an image, and like most applications, the size of your image is determined by the size of your document canvas. 3D is a resolution-independent media, so you can render the same content at many different resolutions and aspect ratios without having to change your assets.
Each Dimension scene has one canvas size that is used for all rendered images. It’s important to note that bookmarked camera views do not retain separate canvas sizes.
When publishing your scene to the web using the Publish 3D Scene (Beta) feature, the canvas size is reflected as an aspect ratio, but the size displayed to the viewer varies for different browsers and screen sizes.