What size balls or scoop do you recommend?

At the 0:53 mark in the video, you can see the baker rolling out a ball of dough in her hands. The ball appears to be about 1.5-1.75" in diameter from my best estimate.

Looking at the recipe, it appears the cookie size is rather flexible so feel free to make the cookies whatever size you desire.

Apparently no one knows- the recipe gives instructions as a rolled cookie, a sliced refrigerator cookie and a drop cookie without mentioning the all important thickness (which matters much more than the overall size). However, it does say it makes 20-30 pieces, which would be a pretty large cookie- probably about thee size of a standard (#5 if I remember aright) ice cream scoop, and it says to flatten only slightly- I'm guessing you want about 1" thick. You might consider the refrigerator cookie option, which would allow you to count the cookies before you actually sliced them.