plot dataframe with two y-axes

Your code is pretty fine. I only add two more lines

 import matplotlib.pyplot as plt 

 df.plot(x="land_cover", y=[1, 2, 3, 4, 5, 6], stacked=True, kind="bar")
 ax = df['size'].plot(secondary_y=True, color='k', marker='o')
 ax.set_ylabel('size')

 plt.show()

In general just add one extra argument to your plot call: secondary_y=['size'].

In this case a separate plot is easier though, because of line vs bars etc.