I think I ran into a similar problem.
See if this helps adjusting the label's font size:
import matplotlib.pyplot as plt
import matplotlib.font_manager as fm
fontsize2use = 10
fig = plt.figure(figsize=(10,5))
plt.xticks(fontsize=fontsize2use)
plt.yticks(fontsize=fontsize2use)
fontprop = fm.FontProperties(size=fontsize2use)
ax = fig.add_subplot(111)
ax.set_xlabel('XaxisLabel')
ax.set_ylabel('YaxisLabel')
.
<main plotting code>
.
ax.legend(loc=0, prop=fontprop)
For the bar width, if your using pyplot.bar it looks like you can play with the width attribute.