If you want to set the figure size once and forever you can store you preferences (not only the figure size) in an initialization file, named matplotlibrc
On linux/Unix like systems
$ mkdirhier ~/.config/matplotlib
$ echo "figure.figsize : 10, 7" >> /home/boffi/.config/matplotlib/matplotlibrc
$ python
...
>>> import matplotlib.pyplot
>>> matplotlib.pyplot.figure()
<Figure size 1000x700 with 0 Axes>
>>> # the above size is in pixels, at the default of 100 dpi
>>>
To know the location of the configuration directory you can do
$ python
...
>>> import matplotlib
>>> matplotlib.get_configdir()
'/home/user1001/.config/matplotlib'
>>>
An initialization file is good for you, but if you want to distribute your script to other people, with different initialization files, well it's no more so good!
A possible remedy is to go with Matplotlib's style sheets
that allows you to load all of your tweakings with a single statement
plt.style.use('./mystyle.mplstyle')
and also the possibility of using a style as a context manager
plt.plot(...) # default style
with plt.style.context('./mystyle.mplstyle'):
plt.plot(...) # your style
so that you can distribute your style sheet along with your script and your user will not be affected by a global modification of the defaults.