The paper size is 15x15 cm (6x6 inches). You can use an A5 format and trim it into a square or simply draw in the center of an A5 paper.
Homework:
1) Draw a sketch following the lesson
2) Take a photo of your sketch and post it in the comments below the video or in your Instagram feed with the hashtag #dreamanddrawschool and tag my account @sketching.beginners
P.S. The lesson will be available for 48 hours, so don't postpone! I believe you'll create a great sketch! Tomorrow I'll make a general review of the works from this lesson!