Keying - Nuke 101: Professional Compositing and Visual Effects - page 327

Now you can see what you are doing and have full control over the size of your
matte. You can increase the Dilate Size property to expand the matte. And then
you can increase the Blur Size property to soften the transition.
4. Increase Dilate1's Size property to 40.
5. Increase Blur1's Size property to 25 ( FIGURE 7.37 ) .
Figure 7.37. An expanded matte.