Be aware that you can resize your canvas to good effect-- while it doesn't create new detail, increasing the file size gives you more "space" to add detail. In fact, a lot of people rough out their images at low-resolutions, and then resize to to work on the fine detail, like the eyes in your case. As a simple rule of thumb, when you resize, double each dimension; this will produce a file four times as large, and isn't always the right answer, but its a start. Resizing doesn't create new detail, but it doesn't destroy it either. If you're interested in digital painting, you might also explore using Painter, which has the ability to replay scripts of your session in more detailed file-- this does add detail.