It appears as though the Resize functionality is only available in the Plus Edition of the software, the documentation can be found at the link below.
Based on the link you provided, it looks like you are using the same version as I am, which is the free one.
https://wpftoolkit.codeplex.com/wikipage?title=ChildWindow%20Plus
Kind of disappointing, we don't utilize the toolkit enough to warrant the cost.