Something similar to the fig.set_size_inches(18.5, 10.5)
of matplotlib.
Or a little bit shorter:
import numpy as np
import seaborn as sns
import matplotlib.pyplot as plt
# some artificial data
data = np.random.multivariate_normal([0,0], [[1,-0.5],[-0.5,1]], size=100)
# plot
sns.set_style('ticks')
g = sns.regplot(data[:,0], data[:,1])
g.figure.set_size_inches(18.5, 10.5)
sns.despine()