Nutz! Coulda sworn the answer was "7".

Lots of good tips, but no specific answer really exists. As was suggested by several above, "best" size would be one that is close to the size that the area will appear in the final render.

Some have suggested a map size twice the resolution that the area will appear in the final render if you want some smoothing, or just nail it at the same size if not.

(clear as mud?)

If your figure's iris is going to cover 50 pixels in the final render, a UV map size that yields an iris area 50 pixels across would give you the most accurate application of the map with the least scaling distortions. However, because the UV template usually combines numerous materials at different scales, it's not likely you'd ever get them all to match without using a different map for each material in the UV template.

Not practical for numerous reasons, so no one really approaches it that way, ...but hey, you asked.

As Carolly suggested, I also suspect that in practice, most folks tend to lean toward "bigger is better", willing to sacrifice performance for detail, even though those details may often not be visible in the final composition.

With all that said, I usually end up making Victoria maps at 2048x2048.