I’m a bit confused by the question. ETS tables don’t survive application restart, so the size it is on shutdown doesn’t affect its size when you restart.

If you have some process for initializing the table then that is gonna be the relevant bit as I see it. You can do it sync or aync to your application boot process depending on whether it makes any sense to have stuff running on a partially initialized table.

Ah, I see. I did not realize that ETS tables are ephemeral (which of course makes a lot of sense).
So: Yes, the ETS table would need to be filled with data before the application could provide most of its functionalities, so it makes sense to do this during the boot process.

Maybe this whole question is somewhat nonsensical, as startup time is obviously something that only happens once, and therefore is much less important than per-request efficiency.

2GB is the limit for DETS, not ETS. ETS tables are essentially as large as you want on a 64-bit system.