change tick mark size

http://matplotlib.org/api/axes_api.html#matplotlib.axes.Axes.tick_params
Example
# Create Figure
fig = plt.figure(1)
ax = fig.add_subplot(111)
#increase size of tick marks
ax.tick_params('both', length=20, width=2, which='major')
ax.tick_params('both', length=10, width=1, which='minor')