StoryNexus games are played over multiple short chunks of text. For most storylets, a single sentence for root and branch descriptions and a paragraph for event text will do fine. It’s especially important to keep text short when the player will read a bit of text multiple times. They’ll probably just skim it after the second time anyway, so there’s no need to write a lot.
That said, the text is one of the major draws of StoryNexus games. It’s a good idea to get players used to the idea that text is a reward for play. So, you might want to make success text a bit longer or more interesting than failure text.
There are some occasions when you want to write more. You might want to go to town on the finale of a story, or describe a particularly memorable scene. It’s not too terrible to do this now and again, but don’t go over the top. Economy of prose is a virtue. If you go over four or five paragraphs, you’ve definitely gone too far.