VisIt provides a default value of 1 for the step length taken when integrating a vector field to create streamlines. The step length is the added to the starting location of the streampoint in the direction of the vector field to give the point in the streamline where the process repeats. The value that you need to provide is very much tied to the spatial and data extents of your data but usually smaller values produce smoother looking streamlines. You should take care not to reduce the step length too rapidly because smaller step lengths also result in streamlines that take longer to calculate. Larger step length values result in streamlines that have visible sharp bends but they are faster to calculate. To change the step length, you type a new length into the Step length text field.