To set the size of a plot during creation, pass figsize=(width, height) to the call, such as

fig, ax = plt.subplots(nrows, ncols, figsize=(10,5))

To readjust the size after the plot has been created, use

plt.gcf().set_size_inches(width, height)

To change the amount of space allocated to axis labels, use e.g.

plt.subplots_adjust(left=0.2, bottom=0.15)

which uses 20% of the plot width for the region to the left of the y axis.