Originally Posted by afv011
Easy. If you double the width and the height (quadruple the pixels), you double the density, so 326dpi.
That is the number for doubling the resolution but not exactly the size of the minimum needed to achieve a retina result. Of course the minimum size depends on several factors such as the distance from the eye to the display and the acuity of the eye itself such as 20/20 or possibly better. See the article on retina display in wikipedia for more details. There have also been web articles verifying and arguing with the Apple definition. A simple search will turn up many of them.