If you would like to insert pre-made eyes into your doll's head as you are sculpting it you will need a pair of glass eyes, beads or polymer clay eyes. These can be made or purchased.
Some doll makers like round eyeballs and some prefer flat backed ones. One technique for round eyeballs is to just roll tiny balls of polymer clay. Eyeballs can be soft fired or completely cured. Then insert the eyes in your doll head as you sculpt the head. The eyes can be painted before the doll head is baked or after. Experiment to find what works best for you.
Another option is to purchase ready made eyes; available in glass or polymer clay. Don't use the acrylic pre-made eyes as they don't take the heat when you bake your doll. You could also purchase white jade or other beads. White glass head pins will also work (not plastic).
So what size is correct for your doll? Click this link and scroll down to Human; the chart has the size in millimeters for various scales: http://www.noadi.net/EyeSizes.html