TO-263S heat dissipation
Surface-mount devices, in general, use a different method of heatsinking from through-hole ones.
You'll notice that one of the terminals of your TO-263 (also called D²PAK, incidentally) package is a very large tab on the back, unlike the others. This tab is bonded tightly to the die, and intended to be used as a heatsink.
What you do for this is use a large copper fill on your PCB (I'm not personally familiar with the calculations that go into how to size it), and use that as your heatsink. You can via-stitch this to a large fill on the other side of the board too, for additional power dissipation. The copper conducts both electric current and heat, and can fairly effectively radiate it out to the surrounding air.
If this still isn't enough, you can get extruded heatsinks intended to be mounted to such a copper fill, but generally if you're thinking of doing that, you may want to consider a different package.