How do I set the figure title and axes labels font size in Matplotlib?

Functions dealing with text like label, title, etc. accept parameters same as matplotlib.text.Text. For the font size you can use size/fontsize:

from matplotlib import pyplot as plt    

fig = plt.figure()
plt.plot(data)
fig.suptitle('test title', fontsize=20)
plt.xlabel('xlabel', fontsize=18)
plt.ylabel('ylabel', fontsize=16)
fig.savefig('test.jpg')

For globally setting title and label sizes, mpl.rcParams contains axes.titlesize and axes.labelsize. (From the page):

axes.titlesize      : large   # fontsize of the axes title
axes.labelsize      : medium  # fontsize of the x any y labels

(As far as I can see, there is no way to set x and y label sizes separately.)

And I see that axes.titlesize does not affect suptitle. I guess, you need to set that manually.

You can also do this globally via a rcParams dictionary:

import matplotlib.pylab as pylab
params = {'legend.fontsize': 'x-large',
          'figure.figsize': (15, 5),
         'axes.labelsize': 'x-large',
         'axes.titlesize':'x-large',
         'xtick.labelsize':'x-large',
         'ytick.labelsize':'x-large'}
pylab.rcParams.update(params)