plt.plot(a, b, 'g', linewidth=3.5, label='a')
plt.rc('text', usetex=True)
plt.legend(labelspacing=1.0, loc=1, prop={'size': 40})
plt.xlabel(
    '{\\fontsize{50pt}{3em}\\selectfont{}a}{\\fontsize{20pt}{3em}\\selectfont{}N'
    )
Multiple fontsize in same label Matplotlib