It depends on the person. I don't think there is a "right size", but I do know that people with extremely large breasts often have back pain and other issues, which may be bad enough that the people opt for breast reduction surgery.
If they are too large to be comfortable for the person who HAS them, then yes, they can be too large. I have several friends who have horrible pain and problems from their breasts and really want reductions.
But from an outsider standpoint? I love all breasts. They are all wonderful.