"Fuzzing" is an automated software testing technique that generates and submits random or sequential data to various areas of an application in an attempt to uncover security vulnerabilities.
For example, when searching for buffer overflows, a tester can simply generate data of various sizes and send it to one of the application entry points to observe how the application handles it.
Make a file where you can put any request into, and the place you want to Fuzz insert the tag <FUZZER>, if you need to count size of data you eg. like in a POST request of a HTTP server, use the tags <COUNT>data<COUNT> and <SIZE>, it could be done as follows:
