I am continually amazed at how many ways there are to do the same thing in matplotlib.
As such, I am sure that someone can make this code much more terse.
At any rate, this should clearly demonstrate how to go about solving your problem.
>>> import pylab
>>> fig = pylab.figure()
>>> pylab.axis('off')
(0.0, 1.0, 0.0, 1.0)
>>> pylab.plot([1,3,1,2,3])
[<matplotlib.lines.Line2D object at 0x37d8cd0>]
>>> pylab.plot([3,1,1,2,1])
[<matplotlib.lines.Line2D object at 0x37d8d10>]
>>> fig.get_size_inches() # check default size (width, height)
array([ 8., 6.])
>>> fig.set_size_inches(4,3)
>>> fig.get_dpi() # check default dpi (in inches)
80
>>> fig.set_dpi(40)
# using bbox_inches="tight" and pad_inches=0
# I managed to remove most of the padding;
# but a small amount still persists
>>> fig.savefig('out.svg', transparent=True, bbox_inches="tight", pad_inches=0)
Documentation for savefig().