Select a drawing from the Drawing Thumbnails panel.
In the Tools toolbar, select the Eraser tool or press Alt + E.
In the Tool Properties view, set the Maximum Size to the desired Eraser size.
NOTEIf you are drawing with a mouse, your strokes will always be at their maximum size. If you are using a pressure-sensitive tablet, your strokes will vary in size depending on the pressure of your pen, ranging between 25% to 100% of the maximum size.