I would say there is no ideal - depends entirely on what you're using it for!

As an example, say you want to view an image on screen at 1000 pixels at the longest side - rendering twice as large and then reducing the size in an image editor (plus doing stuff like sharpening, adding contrast, altering colour etc to taste) can make a render appear much more detailed (advice I picked up from one of blackhearted's pdf's some years ago). If you're going to add painted effects like hair, smoke etc it also makes it far easier if you start with a large render size.

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