I have heard some traders talk about the size of the book that they are trading/managing (or what their "book size" is). How does a trader calculate the size of the book they are trading? I thought it was the notional value of the position(s) that a trader has on at any given time. So, for example, if you are currently long 100 emini S&P contracts then the size of your current book would be roughly : 100 x $70,000 = $7,000,000. Is this correct? Can anyone shed some light on this? Also, what is considered a small book and what is considered to be big book size? Thanks!