These boots from Raychem provide protection to the wires as they enter the connector by providing strain relief and limiting flexibility.  As supplied, they have a cylindrical shape and applying heat (we recommend about 600 F) will get them to shrink down into their final formed shape. We recommend applying ResinTech RT125 to both the connector end and the wire end in order to lock the connector tight to the wire harness.  The table below shows the major diameter (larger end) dimensions of the various boot sizes.

For detailed information see datasheet here