I'm going with A5, which in American standard would be 8.5 x 11.
It also looks as if it has a 1/2" binding, so open surface would then be 8 X 11, which seems to be 'on target'
Okay, I have a large supply of parchment paper, time to get my artistic skills to work and see what I can come up with. The biggest problem for me will be to match the handwriting, which is as much art as anything else.
Also, some of the pics from the screen caps looks significantly different than the flip book on the website. Going with the screen caps.
Stay tuned......