ARPEGGIATOR - Range
The thin blue bar under the steps determines how many steps the pattern will have. To change the number of steps, select the right end of the Range Slider and drag it left or right until the desired number of steps is set.
NOTE - Dragging the Range Slider to the left in order to shorten the pattern does not delete steps - it merely hides them. You can access them again by moving the slider to the right.