If post Downfall Valinor is no longer physically part of Arda, then physical things, the Encircling Sea, the grinding ice of Helcaraxe, no longer define it.
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we are told that it was hidden, but nothing more than that.
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Valinor's status after the downfall is not a case of it merely being hidden. It had been hidden before in the aftermath of the flight of the Noldor.
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The Enchanted Isles were set, and all the seas about them were filled with shadows and bewilderment. And these isles were strung as a net in the Shadowy Seas from the north to the south, before Tol Eressea, the Lonely Isle, is reached by one sailing west.
Silmarillon, Of the Sun and the Moon
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Valinor continues to exist as physical place, requiring physical defences that make it (with a couple important exceptions) inaccesible. After the Downfall, "the world was diminished" as Valinor is no longer physically part of it. Only those with the grace to go there may access it as "all roads are now bent". There may have been original rules regarding its physical boundaries, and it seems that one of the reasons that boundaries existed was to preserve Valinor.
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..the Valar fortified their dwelling, and upon the shores of the sea they raised the Pelori, the Mountains of Aman, highest upon Earth....Behind the walls of the Pelori, the Valar established their domain in that region which is called Valinor;.... in that guarded land the Valar gathered great store of light and all the fairest things that were saved from the ruin....
Silmarillion, Of the Beginning of Days
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The original concept of Valinor is as a sanctuary with well defined boundaries that allow for the defence of that sanctuary from threats from the outside physical world.
However, once Valinor is removed from the world and faces no threats, the idea of well defined guarded sanctuary should no longer hold. I don't know that this means that post Downfall Valinor becomes endless but, to me, once you remove it from the physical world whatever plan there was with respect to its boundaries goes out the window.