From the default project selection, select Create Marker then select the marker size to add to the scene.
Markers can be renamed in the marker list (which is currently only used to identify them in that list).
To rename a marker; highlight the current name by selecting it in the main inspector window and then type the new name to override the current name for the marker.
The marker type and size can be adjusted by:
Select a marker and press Edit Marker Size in the marker selection inspector.
Select the marker size you want.