import numpy as np
import matplotlib.pyplot as plt

plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

x = np.linspace(-5, 5, 100)
y = np.sin(x)

fig, ax = plt.subplots()
ax.plot(x, y, c='r', label='y=sinx', linewidth=3.0)

leg = plt.legend()
leg.get_lines()[0].set_linewidth(6)

plt.show()