What is the recommended length in bytes for the file type "no_ext"?

There`s no correct answer here. It really depends on your application; you cannot add any meaningful numbers there before you have some logs for reference, therefore, keep them as default.

To get started, use the references in the iRules section and write a logging iRule which writes down the values of HTTP Content-Length header as well as the POST payload size. Let it run for a few days, and you will have a good idea what these values are for legitimate requests. For a size limit, I usually 2x the highest values logged.