Part of estimating the size of a story includes identifying the acceptance criteria. These can be created as children of the story page as placeholders, so that the story page becomes a suite. The child scenario tests can be fleshed out as required with plain text during the estimation meeting, and with actual FitNesse tests once the iteration starts. The FitNesse !contents instruction will then list all the acceptance criteria for the story.

For the story page itself, the "as a ... I want ... so that... " template is a good way to summarize the intent of the story.