Whether or not you need a heat sink and/or a fan on the WTC3243 or WTC32ND depends on the maximum voltage drop across the controller and the maximum current supplied by the controller. This can be calculated by plotting these values to determine the Safe Operating Area (SOA) of the controller. The link below will take you to the SOA chart with directions for how to determine whether or not you need the heat sink and/or fan.
Application Note-AN-LDTC01: The Principle of the Safe Operating Area