Firstly, it's hard to know exactly what's happening, without your data, so I had to create dummy data and adjust for your variables, 'self.identifier', and 'xticks' given that we don;t know what those are.
That being said, the function you're looking for is
plt.tick_params(axis='x', which='major', labelsize=__)
as seen in the code below:
import numpy as np
import matplotlib.pyplot as plt
#make dummy data
x=[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40]
y=np.random.rand(len(x))
plt.figure()
plt.plot(x, y)
plt.bar(x, y, alpha=0.2)
plt.title(f"?rednia cena produktu, wed?ug kontrahentów")
plt.xlabel("kontrahent")
plt.ylabel("cena")
plt.xticks(x, [str(i) for i in y], rotation=90)
#set parameters for tick labels
plt.tick_params(axis='x', which='major', labelsize=3)
plt.tight_layout()