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None. Which is why it's the reductio ad absurdem example, but the basic problem of linear perception is real.The encounter distances do help, though I'd say they're equally unrealistic - as you suggest. The longest I saw was plains - max of 1440' (6d6*40). At which point you're never going to be able to see it anyway.
I guess you just roll the encounter distance and say you spot it 6d6*40 = 800 feet away whether it's a colossal titan or a tiny pixie.
Unless it's using stealth, in which case you start from there applying the usual Perception rules.
I think the mistake here is assuming that a Perception check is the only way to sense something in game. This point of view is no doubt influenced by my move to 5e, which is infinity times less granular in its skill rules.