If you use %pylab inline you can (on a new line) insert the following command:
%pylab inline
pylab.rcParams['figure.figsize'] = (10, 6)
This will set all figures in your document (unless otherwise specified) to be of the size (10, 6), where the first entry is the width and the second is the height.
See this SO post for more details. https://stackoverflow.com/a/17231361/1419668