how can i only change the inner radius of a torus(donut) in unity3d?,how can I change only the inner radius of a torus (donut) in unity3D?

I'm trying to develop a 3D eyeball model with unity3D and I'd like to make the eyeball tweakable, one of the function is to input the size of the pupil and iris model will be changed to the right size. How can i achieve this kind of function in unity3D?,I'm trying to develop a 3D eyeball model with unity3D and I'd like to make the eyeball tweakable, one of the function is to input the size of the pupil and iris model will be changed to the right size. How can i achieve this kind of function in unity3D?