You are probably looking for this: http://matplotlib.org/users/text_props.html
For instance you can add
ax.text(bar.get_x()+bar.get_width()/4., 0.65*height, '%.2f'%float(height), rotation='vertical', horizontalalignment='left', va='bottom', weight='bold')
To use a bold font. You can use fontsize=somenumber to change the font size.