According to the Dictionary of Word and Phrase Origins, foolscap is a term used to designate stationary.
Foolscap stationary measures from 12 by 15 inches up to 13-1/2 by 17 inches. The most common size is 13 by 16 inches, which is often folded to make pages of 8 by 13 inches. Originally it was a printing paper used in England and got its name from the ancient watermark of a fool's head and cap used to identify the paper. The earliest specimens of paper using the foolscap watermark date back to before the time of Shakespeare.
So, if you want to channel Shakespeare in your writing, try to find some foolscap to write on. It might be fun to use pen and paper, for those who only use a computer.