The official word from Parallels support is: "Unfortunately, currently customer resolutions are not available in Parallels Desktop. Since then, only resolutions listed under 'vga=ask' are available for use."
So, if you're looking for a virtual environment for testing Android apps (or any other OS not supported by Parallels Tools) using any specific resolutions not listed in that screenshot, including but not limited to any portrait resolutions, Parallels simply won't do it. VirtualBox is probably your best option, and android-x86 seems to run quite nicely under VirtualBox 5.2.
As a long-time Parallels customer, it's very disappointing to learn that a free virtualization app offers a superior feature set, especially in something so basic as screen resolution.
Last edited: May 16, 2018