Try figsize param in df.plot(figsize=(width,height)):
df = pd.DataFrame({"a":[1,2],"b":[1,2]})
df.plot(figsize=(3,3));
df = pd.DataFrame({"a":[1,2],"b":[1,2]})
df.plot(figsize=(5,3));
The size in figsize=(5,3) is given in inches per (width, height)
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.plot.html