The Marker size option allows you to roughly match the size of the shot marker to the size of your bullet hole. Once you select a marker size, that selection is saved as the default marker size until you change it again.
Mark Group**:** The mark group feature is intended to help you mark prior strings of shots to track your groups or to keep track of different shooters. In the camera view, tap the icon on the bottom right of the screen. A pop up will display size and color. Choose the desired size and color of your markings. Future changes to your selection will only apply to the markings after you have made your selection. The previous markings will not change.