The size of the heatsink base must be the package body plus the area needed for the attachment at a minimum. See Heatsink Attachment and Mounting for more information on heatsink attachment. As noted in Coplanarity and Etching of the Contact Surface Area, the package body can be larger than the surface area. Refer again to the mechanical drawing and the parameters labeled D and E that pertain to the package body size. As indicated in the following figure, use the D and E parameters from the top view of the mechanical drawing to determine package body size.
It can be sized larger if more cooling is necessary. The thickness of the heatsink base depends on several factors including the material used, necessary rigidity of the heatsink, the amount of heat spreading necessary, the type and attachment of fins, the use of 2-phase cooling, and other factors.