The question of the resurrection of aborted children hinges on our interpretation of human life. If these are lives, they will be resurrected. If they are not, then the question is moot. Augustine's point is that either way it is no objection to the doctrine of the resurrection.
As to whether infants' resurrected bodies will be those of adults or of infants, Augustine holds that the main thing is that they will not be less than they were in this life.
Really, we'll probably all have bodies the size they were in our prime, or what would have been our prime for those who died as infants.
Being conformed to the image of God does have a physical component to it (especially in the resurrection), but it is really a spiritual statement. We are being made holy, so we need not quibble over this point.