I found this https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html
b: Controls to what degree document length normalizes tf values. The default value is 0.75.
I think this is what I want.
Come back to the "norms" setting approach, seems it not only disable field length norm, but also disable all other normalization factors. So if I just want to disable field length norm, I should use custom similarity with b=0.
Can any expert confirm?