Shrinkage is how much an item made from clay or other material will get smaller - sometimes this is a big deal, if you're making something that is supposed to fit another piece, such as a lid.
Luckily, with Hypertufa, what you see is what you get! There is no shrinkage, and no need to adjust the sizing to allow for it.
That's the good news.
However, just because the overall piece doesn't change, the thickness of the walls does, and therefore, you should make the sides and bottom of a larger piece thicker.
The hypertufa cures very hard, but it's still brittle if it's too thin, and you run the risk of shattering it if it's dropped on a hard surface.
This is something that you should test, based on your recipe and your conditions for curing.
Hypertufa is often meant for very rustic types of projects where size of the finished piece is not important. If accurate sizing is crucial to the success of your project, think about using some other material to make it out of.