import matplotlib.pyplot as plt
import figure_scale as fs
figsize = fs.FigureScale(4.0, 2.0, units="in")

plt.rcParams.update({'figure.figsize' : figsize})

fig, ax = plt.subplots(figsize=figsize)

with figsize():
    fig, ax = plt.subplots()

@figsize()
def my_plot():
    ...