If you're more used to using ax objects to do your plotting, you might find the ax.xaxis.label.set_size() easier to remember, or at least easier to find using tab in an ipython terminal. It seems to need a redraw operation after to see the effect. For example:

import matplotlib.pyplot as plt

# set up a plot with dummy data
fig, ax = plt.subplots()
x = [0, 1, 2]
y = [0, 3, 9]
ax.plot(x,y)

# title and labels, setting initial sizes
fig.suptitle('test title', fontsize=12)
ax.set_xlabel('xlabel', fontsize=10)
ax.set_ylabel('ylabel', fontsize='medium')   # relative to plt.rcParams['font.size']

# setting label sizes after creation
ax.xaxis.label.set_size(20)
plt.draw()

I don't know of a similar way to set the suptitle size after it's created.

You can also do this globally via a rcParams dictionary:

import matplotlib.pylab as pylab
params = {'legend.fontsize': 'x-large',
          'figure.figsize': (15, 5),
         'axes.labelsize': 'x-large',
         'axes.titlesize':'x-large',
         'xtick.labelsize':'x-large',
         'ytick.labelsize':'x-large'}
pylab.rcParams.update(params)

Functions dealing with text like label, title, etc. accept parameters same as matplotlib.text.Text. For the font size you can use size/fontsize:

from matplotlib import pyplot as plt    

fig = plt.figure()
plt.plot(data)
fig.suptitle('test title', fontsize=20)
plt.xlabel('xlabel', fontsize=18)
plt.ylabel('ylabel', fontsize=16)
fig.savefig('test.jpg')

For globally setting title and label sizes, mpl.rcParams contains axes.titlesize and axes.labelsize. (From the page):

axes.titlesize      : large   # fontsize of the axes title
axes.labelsize      : medium  # fontsize of the x any y labels

(As far as I can see, there is no way to set x and y label sizes separately.)

And I see that axes.titlesize does not affect suptitle. I guess, you need to set that manually.

7 (best solution)

 from numpy import*
 import matplotlib.pyplot as plt
 X = linspace(-pi, pi, 1000)

class Crtaj:

    def nacrtaj(self,x,y):
         self.x=x
         self.y=y
         return plt.plot (x,y,"om")

def oznaci(self):
    return plt.xlabel("x-os"), plt.ylabel("y-os"), plt.grid(b=True)

6 (slightly worse solution)

from numpy import*
M = array([[3,2,3],[1,2,6]])
class AriSred(object):
    def __init__(self,m):
    self.m=m
    
def srednja(self):
    redovi = len(M)
    stupci = len (M[0])
    lista=[]
    a=0
    suma=0
    while a<stupci:
        for i in range (0,redovi):
            suma=suma+ M[i,a]
        lista.append(suma)
        a=a+1
        suma=0
    b=array(lista) 
    b=b/redovi
    return b



OBJ = AriSred(M)
sr = OBJ.srednja()

An alternative solution to changing the font size is to change the padding. When Python saves your PNG, you can change the layout using the dialogue box that opens. The spacing between the axes, padding if you like can be altered at this stage.

Place right_ax before set_ylabel()

ax.right_ax.set_ylabel('AB scale')

To only modify the title's font (and not the font of the axis) I used this:

import matplotlib.pyplot as plt
fig = plt.Figure()
ax = fig.add_subplot(111)
ax.set_title('My Title', fontdict={'fontsize': 8, 'fontweight': 'medium'})

The fontdict accepts all kwargs from matplotlib.text.Text.

Others have provided answers for how to change the title size, but as for the axes tick label size, you can also use the set_tick_params method.

E.g., to make the x-axis tick label size small:

ax.xaxis.set_tick_params(labelsize='small')

or, to make the y-axis tick label large:

ax.yaxis.set_tick_params(labelsize='large')

You can also enter the labelsize as a float, or any of the following string options: 'xx-small', 'x-small', 'small', 'medium', 'large', 'x-large', or 'xx-large'.

libraries

import numpy as np

import matplotlib.pyplot as plt

create dataset

height = [3, 12, 5, 18, 45]

bars = ('A', 'B', 'C', 'D', 'E')

x_pos = np.arange(len(bars))

Create bars and choose color

plt.bar(x_pos, height, color = (0.5,0.1,0.5,0.6))

Add title and axis names

plt.title('My title')

plt.xlabel('categories')

plt.ylabel('values')

Create names on the x axis

plt.xticks(x_pos, bars)

Show plot

plt.show()

If you aren't explicitly creating figure and axis objects you can set the title fontsize when you create the title with the fontdict argument.

You can set and the x and y label fontsizes separately when you create the x and y labels with the fontsize argument.

For example:

plt.title('Car Prices are Increasing', fontdict={'fontsize':20})
plt.xlabel('Year', fontsize=18)
plt.ylabel('Price', fontsize=16)

Works with seaborn and pandas plotting, too!