How do I change the size of the picture made of matplotlib?

If you already have an image file, You can resize the canvas in the following ways:

import matplotlib.pyplot

fig = matplotlib.pyplot.gcf() #Bring into the current figure
Fig.set_size_inches (18.5, 10.5) #Size (in inches)
figure.savefig('test2png.png',dpi=100) #SaveCurrentFigure

set_size_inches(w, h, forward=False) changes the size of the figure in inches. When set to forward=True, the canvas size is automatically updated (reflected directly from the gui window you are using).