Criteria for the numbering of human chromosomes
Why do you think it was "fixed?"  Here's a nice review of the history of human cytogenetics, which included not only the original image from 1956 but points out a report which comments on the standardization of chromosome number.  The autosomes were indeed numbered by length, and the sex chromosomes are traditionally put at the end as they are "numbered" 23 but clearly function quite differently.  Gene content was decades away from being known at the time, and honestly isn't even known now.  It's also just as arbitrary; simple size is easy enough and makes for rather nice pictures.