Unless you want head size to always activate that particular morph(s) it's best to create a new master that links head size with a particular morph. This is much more flexible and less invasive then attaching all or nothing to head size.

So

New master

Start teaching

Set new master to 1

Set head size to 2 (or whatever)

Set another morph to whatever value

0 new master and slaves back out. Stop teaching.