I typically recommend the small (size 5 inch) for newborns to a year. The small size is also nice because you can easily fit it in your diaper bag. These are just the right size for little hands to grab and roll.
This little cutie is kicking around a small size, and he is 4 months old in this video.
The medium size (7 inch) is great for a one year old, and is really fun for floor play. Little ones who are just learning to walk will find it especially fun to kick around and chase. It is also good for learning to take turns when you roll the ball back and forth to one another.
The large size (9 inch) is especially fun for toddlers and big kids. It is close to basketball size and is just right for learning to catch and throw. The best part-it doesn't hurt a bit if you miss!
This is my little sweetie with a large size ball. He was 18 months old in this picture.
Of course, you really can't go wrong with a ball in any size because everybody loves a ball!