Hatari has some limits on what sizes can be used. They're set based on what different TOS versions and what "commonly used" GEM programs work with.
Test before trusting GEM programs to work in VDI mode without crashing with your unsaved data. Some of them can do unsafe assumption about screen sizes and what kind of buffer sizes are enough for screen related operations.
As to Falcon, there are several options: * EmuTOS 512k: EmuTOS supports Hatari VDI modes also on Falcon (EmuTOS has no DSP nor true color mode support yet though) * TOS v4: just use one of the native Falcon utilities to set up suitable Videl resolution at boot. For example, with Videl Inside (v1.2), you can set up Falcon to boot up with (up to) 1664 x 592 sized 256-color resolution, or 832 x 592 "true color" resolution