Is 160 Words a Reasonable Length for Student Forum Postings?

Each week I ask students in an online forum to make at least three postings, for which they receive up to 2% marks. The students ask how long the postings should be and I struggle to answer this. I have tried saying "one or two paragraphs", but students seem to think more is better. I have tried marking them down for writing too much, but then they ask how much is too much. Moodle now has a word length function for forum postings, so perhaps I can tell them a number of words, but how many words?

Australian universities typically set 50 to 60 words per percent of assessment. So that would be 100 to 120 words for 2% of assessment. But I am asking the students to make at least three postings per week, so they would be on average at most 40 words, which seems a bit short.

In "Student-led facilitation strategies in online discussions",  Evrim Baran and Ana-Paula Correia report students posting an average of, 96, 150 and 181 words for different discussions. Nirmaldasan in "Plain Paragraph Length" suggests typically 100 to 150 words, but as short as 60 words. So my "two paragraphs" would translate to 120, 200, or 300 words.

Based on this, 160 words might be might be reasonable length. This also happens to be about the number of words which fit on a standard definition TV screen (as used for Teletext).