Can I set a virtual screen’s reported width and height independently?

The virtual-screen definition provides Reported diagonal size, rather than independent reported width and height. BetterDisplay derives its reported millimeter dimensions from that diagonal and the configured pixel geometry. Changing the diagonal does not introduce a different pixel aspect ratio.

Open Settings > Displays > [virtual screen] > Additional Resolution Settings… to find Reported diagonal size. This setting requires Pro. Reconnect the virtual screen when prompted to apply a change. It describes the virtual display to macOS. It does not change your physical monitor or stretch the image to match a panel with different physical proportions.

A connected display also has Display physical dimensions under its General Settings > Additional Settings…. Those width and height values serve a different purpose: BetterDisplay uses them for UI-scale matching among display-group members. They do not replace the virtual screen’s reported dimensions or change its desktop pixels.

If your goal is to stretch the displayed image, use image-fitting options such as Free Aspect Ratio in a stream or Picture in Picture view. That changes the presented image, not the virtual display’s physical-size report. See stretching a desktop for this separate workflow.

See Virtual Screen settings and display physical dimensions for the two kinds of size settings.