After the OS is booted, you can run VideoRep; at the command line to retrieve a list of screen resolutions supported by your target hardware/VM. By default, ZealOS uses the standard resolution of 1024x768, however any of the resolutions listed can be used as a screen resolution for ZealOS.
If you are using the Limine bootloader, you need to modify /Boot/Limine.CFG and change the value in the RESOLUTION field. (Note that INTERFACE_RESOLUTION only affects the resolution of the Limine bootloader itself, the RESOLUTION value applies to the OS.)
If using the traditional Public Domain BIOS-mode bootloader, you will need to edit /Kernel/KStart16.ZC and change the values in REQUESTED_SCREEN_WIDTH and REQUESTED_SCREEN_HEIGHT. Then you can recompile the kernel with BootHDIns; following the instructions on the wiki.
Reboot to see the new resolution settings take effect.