What is a typical Notability's file size? I don’t think I can research from the vendor, and I want to plan if I choose to use this app. 

What if the notes contains that 20-minutes lecture/audio clips - what is the size for each of these attachments?
I can't say for sure but my whole Notability folder is less than 300Mb and it includes over 300 notes including pictures, pdfs, text books, etc.