This feature does not exist for discussions. The only place I have seen an enforceable word count is for essay questions in New Quizzes: How do I create an Essay question in New Quizzes?
About the only thing an instructor can do with discussions is convey the importance of having the proper length and then make it a factor in the grading so that students learn it is important.
The rest of this is an explanation of what it would take for your institution to do this since it is not available in Canvas. For more people, I recommend the technique in the previous paragraph of letting students know the importance. If you are using Canvas Free for Teachers, then communication with the students is your only real option.
There would need to be some way to configure it as well. That means writing an interface on top of the Canvas interface or or allowing it to be specified within the post itself. As an instructor / administrator / programmer, I would take that second approach. You cannot leave comments within the HTML, but we have determined that some of the data attributes remain. Instructors would have to edit the HTML and add an element that would let the system know that it should enforce a word count and what the minimum word count was.