To begin with two comments;
1) a Balmoral is a beret with a PomPom on top the basic knitting instructions are the same and the size very greatly.
2) Stop over at Xmarksthescot.com, this is a forum for those that wear kilts and this question comes up once or twice a year.
It is my understanding that the size varied based on personal taste.
You can also get a pattern for Diane at Russel Lodge Custom Knits