Rule of Thumb for PCB pad sizes of SMD devices?
The drawing in the O.P. comes from a datasheet.  Some datasheets have a dimensioned drawing for the footprint (land pattern).  Here's an example.  Notice that the pads (top right corner) a somewhat wider than the legs.


Originally from here.  Found this by googling SOT-223.

SOT-223 is a fairly common package.  It's likely that your layout software has a standard footprint for it.  There's a good chance that it's accurate.