Spread two fingers apart to zoom in up to 10x, and move two fingers to change the area shown.
Use the slider that appears while "Brush" or "Restore" is selected (the one with a small dot at one end and a large dot at the other) to change the thickness of the brush. The size you set is kept for next time.
If your finger hides the spot and makes it hard to see, use "Touch Offset" in Settings to shift the erasing point above your finger.