You can change the maximum method/function length warning behavior in NetBeans options (it’s under Tools->Options->Editor, in the Hints tab, under “Too Many Lines” section in the checkboxes list).
There you can set the maximum number of lines, how you would like to be warned, etc.
I believe that the rule was introduced by NetBeans developers because when working in teams, the automated tools that QAs use to “inspect” code flag long method declarations/functions bodies. Unfortunately, the use of automated tools by “code analysts” is on the rise, whilst their understanding of the reasons behind that are still limited. I do not say that your functions should be hundreds of lines long – that’s just plain wrong, but a hard-coded number as a coding law – come on!