Learn English – What does it mean to “feel Humpty”

If something is "made to order", it means it is:

Made in accordance with particular instructions or requirements.

The interpretation of this particular question is tricky however. The three commas in the first part make it difficult to know without further context how to correctly parse it.

My interpretation:

The book in question was being refered to as being "[not exactly] written to order". In other words, it was not exactly written for one particular girl, although the author clearly had the girl at the forefront of her mind when she wrote.

Alternative interpretation (supplied by Paul Richter)

The author was not instructed or required to write it, she wrote it (for the girl) of her own accord.

The question is who she is saying she has made the book "to order" for.

Perhaps reading on in the book would clarify this.

Very plausibly sizes : 5 feet 10 inches, and 120 pounds. Models are notoriously required to be tall and thin.

EDIT In metric that's around 1.78 m and 54 kg.