figsize in matplotlib is not changing the figure size?

One option (as mentioned by @tda), and probably the best/most standard way, is to put the plt.figure before the plt.bar:

import matplotlib.pyplot as plt

plt.figure(figsize=(20,10)) 
plt.bar(x['user'], x['number'], color="blue")

Another option, if you want to set the figure size after creating the figure, is to use fig.set_size_inches (note I used plt.gcf here to get the current figure):

import matplotlib.pyplot as plt

plt.bar(x['user'], x['number'], color="blue")
plt.gcf().set_size_inches(20, 10)

It is possible to do this all in one line, although its not the cleanest code. First you need to create the figure, then get the current axis (fig.gca), and plot the barplot on there:

import matplotlib.pyplot as plt

plt.figure(figsize=(20, 10)).gca().bar(x['user'], x['number'], color="blue")

Finally, I will note that it is often better to use the matplotlib object-oriented approach, where you save a reference to the current Figure and Axes and call all plotting functions on them directly. It may add more lines of code, but it is usually clearer code (and you can avoid using things like gcf() and gca()). For example:

import matplotlib.pyplot as plt

fig = plt.figure(figsize=(20, 10))
ax = fig.add_subplot(111)
ax.bar(x['user'], x['number'], color="blue")

Try setting up the size of the figure before assigning what to plot, as below:

import matplotlib.pyplot as plt
%matplotlib inline  

plt.figure(figsize=(20,10)) 
plt.bar(x['user'], x['number'], color="blue")