How to print screenshot with "fit to page" option?
There is not a way to do this in Grab.  However, if you use Preview, you can do the same thing as Grab, by going to File > Take Screen Shot > (Selection).  You can then use the scale to fit page function, as shown below.  Preview has many hidden and cool features that not many people know about.  You don't really need to use Grab, when you could just use Preview, and get what you want!  Hope this helps!

(Screenshot taken with Preview)