Sorry for the delay in responding. The sizes are listed in the appearance.2da, in the SizeCategory column - medium is 3. You can view the appearance.2da using NWExplorer, found here. It will display in NWN HAK Files/cep2_custom.2da/Game Data/appearance.2da - you can export to text.
In this case the book is appearance 1437 (you'll have to ask a dm about various creatures if you're not proficient enough with the toolset to view appearances in it). It's size 2, or small, and not an option.