I haven't been doing many of these lately, but while I was making thumbprint guestbooks, I put together a helpful diagram of how many thumbprints would fit on different standard sizes of paper. I hope this helps clients determine what size guestbook they need for their event.
It may be helpful to click on the image above to expand it.
Here's another way of looking at it:
And the same progression again, but vertical (sooo pinnable):
Hope this is helpful to anyone who's considering a thumbprint guestbook tree!