This is an idea I learned from Mario Rinvolucri at one of the Pilgrims courses (and I am almost sure it can be found in one of his books on grammar games).
Shrinking or expanding sentences involves a lot of knowledge of grammar, structures and vocabulary to be able to determine the most important parts a story can be reduced to. Because it is a challenging task, my students always loved it and participated with eagerness.
Also, limiting the target number of words to exactly six, not more and not less, makes this task even more difficult.
If you have time, you can play with the shortened or expanded stories your students produced so that they have to add/take two consecutive words, take one out and replace it with two or any other similar rule that makes sense in your context.