So, he calculated the max mass of snake that would fit in a 10' cube.
Then he got the lower and upper bounds (500lbs and 2000lbs) from the book's description.
Then got the length's of snake that correlate to those weights.
Therefore the "answer" to "how big a snake can I shapeshift into?" is:
"Practically, between 20 and 40 feet. However, theoretically a snake of 80+ feet could fit into a 10' cube"