I usually care more about the relative sizes of height/width rather than the actual values in inches. So, I use plt.figaspect()plt.figaspect() to set the aspect ratio.
# Make the image 3x as wide as it is tall
fix, ax = plt.subplots(figsize=plt.figaspect(1/3))
plt.title("I'm 3x as wide as I am tall")
plt.show()
