Use matplotlib function setp according to this example:
import seaborn as sns
import matplotlib.pylab as plt
sns.set_style("whitegrid")
tips = sns.load_dataset("tips")
ax = sns.stripplot(x="sex", y="total_bill", hue="day", data=tips, jitter=True)
plt.setp(ax.get_legend().get_texts(), fontsize="22") # for legend text
plt.setp(ax.get_legend().get_title(), fontsize="32") # for legend title
plt.show()
Another way is to change font_scale of all graph with plotting_context:
http://seaborn.pydata.org/generated/seaborn.plotting_context.html