Different size meet your different needs
1.6 X 9 inch & 3 X 12 inch & 4 X 16 inch auger drill bit
1.6 X 9 inch is a good size for root aeration, and for installing smaller plant stakes or poles.
3 X 12 inch is a good size for horizontal drilling, mixing paint, cement or soil.
4 X 16 inch is a good size for setting smaller fence posts or planting annuals and perennials that come in quart-sized pots.