As big as possible 
The issue is that this will be a trade off between component sizes, overall device size and the battery size. The hardware designers are now working on two approaches and we need to see where they and up with.
How thick a device would you still accept? This is basically the biggest question here. Battery capacity is in direct proportion to its physical volume. The more capacity the more volume = space you need. Since we use modules for baseband and WiFi and these take up significant space and since we use the iMX8M which is not available as POP (package on package) our PCB will become pretty large. If you want to have the battery side by side with the PCB this leaves little room for the battery. That’s why are now looking into having the battery kind of sandwiched between display and PCB so that we have the full area for the battery. The capacity becomes a question of thickness only.
So how thick a device would be acceptable for you? 10mm? 11? 12? 15? 20?
You point out a number and we can basically tell which capacity this will result in - almost at least.
Cheers
nicole