Matplotlib make tick labels font size smaller

To specify both font size and rotation at the same time, try this:

plt.xticks(fontsize=14, rotation=90)

There is a simpler way actually. I just found:

import matplotlib.pyplot as plt
# We prepare the plot  
fig, ax = plt.subplots()

# We change the fontsize of minor ticks label 
ax.tick_params(axis='both', which='major', labelsize=10)
ax.tick_params(axis='both', which='minor', labelsize=8)

This only answers to the size of label part of your question though.