Keep hydrated, eat a lot of healthy food, and take your vitamins including iron. It's not the breast size that matters, it is your ability to produce milk. This is also affected by the baby sucking, so nurse a lot.
Most women can breast feed no problem. :>) You will make adequate milk for your baby most likely. If baby doesn't seem satisfied, talk to your pediatrician. But overall, I would expect the baby to be. Drink plenty and get rest. When is baby number two due??