I would venture out to say that 600km x 600km you are pretty screwed.
Max unreal size (effective) is like 16km x 16km. Now, if you level stream you maybe able to get away with it. Your going to be looking at chopping that thing into like 100 squares tho and gettign your landscape culling “just right”. Are you guys able to load everything @ the 600x600 size in a single mesh ?