Questions everyone and I think is of interest of the majority! "Maybe"

I screen cap this image. Though is a small image, but I wonder, if I am to create an image like that, where the head shows only a tiny fraction of the entire image size, what size should I aim for and with excellent results.
I understand that for close shots, like showing big portion of the head, higher map size is better, but for that size where the head only represents a fraction of the image size, what image size would you create in Zbrush for that model as shown here?

This is not a zbrush specific question as I think is relevant to the final rendering overall look.