No matter how I scale the Minecraft interface or what I set the book GUI to in the mod config, the book always displays at what I figure is the minimum size. I'm using the Guidebook-1.10.2-1.6.0 release.

I'm having the exact same problem. :\

Have you tried (in Minecraft settings) going to:
Options
Video Settings
and setting "GUI scale: Auto"

That seemed to work for me. Hope it helps.
Also the gbook.cfg I have is this:

general {
    # -1 for same as GUI scale, 0 for auto, 1+ for small/medium/large
    I:BookGUIScale=-1
}

There seems to be a bug with the "same as GUI scale" setting. Looking into it.

Nevermind, it's due to the book's choice of text scaling: A book with font size = 0.6, will have book scaling=1 on all 3 of small/medium/large, since 3*0.6=1.8 rounded down, is still 1. The solution is to manually specify a book scaling value 2/3/whatever in the config.