Why is my preferred resolution available only in LoDPI?

A HiDPI desktop needs a larger rendering surface than the same desktop size in LoDPI. Your Mac can therefore support a display’s physical output resolution while being unable to render every desired scaled desktop in HiDPI. Adding a custom size or buying Pro does not remove those system limits.

For the rendering background, see how macOS scaling, HiDPI and LoDPI work.

For the usual two-times backing scale, a desktop that looks like 3840 × 1080 needs a 7680 × 2160 rendered image. A 5120 × 1440 HiDPI desktop needs 10240 × 2880 rendering pixels, even when the physical monitor itself is only 5120 × 1440. See HiDPI and Resolution for the distinction between desktop size, rendering size and the final display signal.

A smaller logical desktop does not necessarily mean a lower-resolution signal. For example, a 2560 × 1080 HiDPI desktop renders at 5120 × 2160 and can map directly to a 5120 × 2160 panel while keeping icons comfortably large. Check the actual output in Display Information. You do not need to select a 5120 × 2160 logical desktop just to use that panel’s physical pixels.

Find the relevant limit

Open Settings > Displays > [display] > Display Information… and look under Capabilities for Horizontal GPU resolution limit and Allocation limits. These rows appear only when the system supplies the data. They distinguish reported HiDPI sizes from their corresponding LoDPI dimensions.

Limits can depend on the Mac, connection, display arrangement and macOS. An advertised ability to drive an 8K display does not prove that every scaled HiDPI desktop is available on every port. A larger cable-bandwidth rating also does not bypass a rendering or allocation limit.

First confirm that your custom configuration has been applied and activated. Then try a lower HiDPI desktop size. If you have suitable alternative connections, comparing a direct connection with a dock, or another port, can show whether the available mode list differs. Treat the observed modes as the result for that setup, rather than assuming one port type is always better.

If you need a deliberately small desktop, enable Enable lower than 720p resolutions under Settings > Displays > Overview > Advanced Display Settings…. Accept the offered virtual-screen reconnect when you want the changed mode list applied to connected virtual screens. That confirmation cycles all connected virtual screens. This removes an app-side size restriction, not every macOS limit, and interface elements may no longer fit.

Limits also apply at very small desktop sizes. A listed HiDPI mode with a very low vertical dimension can be rejected by macOS and revert immediately. Showing the mode or adding a custom size does not bypass that restriction, and a virtual screen is not a guaranteed escape. Use a nearby working size. The minimum usable height depends on the configuration.

If native scaling cannot provide the size

A virtual screen mirrored or streamed to the physical monitor can offer a different set of desktop sizes. Start with Create Virtual Screen and match the monitor’s aspect ratio. The virtual screen supplies the desktop. The physical monitor displays its image.

For a virtual screen that needs a rendering dimension above 7680 pixels, enable Enable resolutions over 8K (4K HiDPI) in its Virtual Screen settings, unless the global extension is already enabled. This permits larger virtual modes. It does not raise the native display pipeline’s hardware limit. A 4608 × 1920 HiDPI desktop, for example, requires a 9216 × 3840 rendering surface.

This is a workaround with different behavior and performance. Test text clarity, refresh behavior and sleep/wake before depending on it. Enabling larger virtual resolutions does not increase the native connection’s limit or guarantee that an oversized desktop will work well. A lower native HiDPI size may be the more comfortable choice.