pgfplots determine the default size (width and height) of a plot particularly the box defined by axes?
I have checked one of the plots that I generated with default values of pgfplots using trial-and-error and draw the line:
\draw (0,0) -- (6.86,5.69);
It seems like, the inner box is approximately 6.86cm by 5.69cm. Or 194.457pt by 161.291pt.