Shouldn't have anything to do with database size, no - SQLite is very lean in its RAM usage (only brings a little piece of the database into memory at a time) and in any event nothing in Settings / Flashcards ought to cause any extra database data to be reloaded since it's actually just adjusting in-app settings relating to flashcards.
Interesting, good to know.
It could be something iCloud related, since even if you're not using iCloud sync the app checks to see whether you have a dormant iCloud database when you enter that screen - does Pleco have permission to use iCloud storage? (if so, does it get rid of the crashes if you turn that permission off?)
I turned that permission setting off, and no crashes so far. I will have to give it some time and will get back to you in case I still get the error.