The range of options under Page Format is far too limited, and wildly eccentric. Has anyone actually ever released an A0 game, compared to the thousands at 9"x6" or 8.5"x5.5", which are not available as sizes?
For what it’s worth, 5.5"x8.5" is Half Letter, and 6"x9" is US Royal, both of which are on the list.
As @dodgepong indicated, we do have 9"x6" or 8.5"x5.5" as size options, just using their standard names - US Royal and Half Letter. They’re both very common sizes, so we need to include them.
For the ISO 216 (A#) sizes - we include from A0 since it’s entirely plausible for someone to publish something on a large poster size like that. It’s not actually uncommon for stock artists to publish high-DPI versions of their work that are available at A0 or A1 size.
If there are any sizes missing from the list, please let us know and we’ll add them!