COMPLICATED OVERVIEW
The human iris is about .55" (13-14mm) in diameter, but different sculptors sculp the eyeball differently and at the smaller scales even the slightest variation can be troubling. It makes no difference how open the eye is, the width does not change, so to determine the correct diameter of the iris for your particular figure, measure the width of the eyeball and divide that dimension by 2. In other words, if the width of the sculpted eyeball is 4mm, the correct size of the iris should be 2mm.

THE SIMPLE SOLUTION
To eliminate the tedious measuring and guess work, we have introduced individual sheets of irises for specific scales from 1/35 to 1/4 scale HERE. Choosing one of those will, in 99% of the cases, be correct. If you cannot find what you need there, here is a list of iris diameters found on the following sheets. If you do not see the exact size you need, use the next larger size.
Bigger is better.

AR99001- .032" (.81mm), .038" (.96mm), .044" (1.12MM), .051" (1.3mm)
AR99002 - .060" (1.5mm), .063" (1.6mm), .070" (1.78mm), .075" (1.9mm)
AR99003 - .084" (2.1mm), .104" (2.6mm), .126" (3.2mm)
AR99003A - .052" (1.3mm), .061" (1.55mm), .079" (2.0mm), .097" (2.5mm), .127" (3.23mm), .150" (3.8mm)
AR99003B - .052" (1.3mm), .061" (1.55mm), .079" (2.0mm), .097" (2.5mm), .127" (3.23mm), .150" (3.8mm)
AR99064 - .072" (1.83mm), .084" (2.13mm), .120" (3.05mm)