set_size_inches(width,height)
As of Matplotlib 2.0.0 it will even automatically update your canvas as the forward keyword now defaults to True. (For earlier versions you need to specify forward=True explicitly.)
If you want to just change the width or height instead of both, you can use
set_figwidth
or
set_figheight
Unfortunately you need to specify forward=True to automatically propagate the changes as it still defaults to False for these functions. (And these functions don’t even support forward in versions older than Matplotlib 1.5.0)