After some testing, I got the same numbers 8796093022208.0 as @Rynex_FNC
To reproduce:
Create blank level
Go in Content Browser and select all FN devices
Drag them into level and Save All
Restart UEFN and reopen project
LevelBounds will be huge 8796093022208.0 x 8796093022208.0 x 8796093022208.0
After deleting all devices/actors and restarting UEFN again, LevelBounds stay massive.
I would like to know if huge LevelBounds like this can affect performance and should I delete them?