The girl on the beach demo was about 12 sq. feet - 36X48 inches, so the math is it is about 4-5 20X20 inch ones...

However I did refined my technique and that is what I teach in class - fresco helps you to cut all redundancies and insertanties in painting that "paint over mediums" are often trapping us into, since it is so "easy" - make a wrong stroke - not a biggie, just paint it over... so this is why we end up painting one painting for weeks and more....

anyway the old masters giornatas were proportionate to how much they could paint in one day - about 12 square feet - 1 hour per foot... It takes double that time to paint a 1 sq. foot tile simply because you have to let the paint/water "sit" so you do not overwet and wash away the layers you put on already.

buy this book, it has diagram of Sistine Chapels giornatas, the largest one, i believe is over 80 sq feet...

http://truefresco.org/bookshop/4_1000_0810938405_The-Sistine-Chapel-A-Glorious-Restoration.html

PS. You board looks impressive, I assume it is very light?