Custom resolution for animation

Hi all,
I’m working with animations and have a resolution issue.

It is not possible to set a custom resolution using this method

In a floating viewport the horizontal size can be customized, and is NOT limited by the monitor resolution, but the vertical one IS limited.

Thus, I can’t create high resolution animations.
Is this a bug? Is there a workaround?

I’m using the latest build (6.15.19092.12046) + MacOS 10.13.6