how can I change those things to get what I want?

Hi, so I got a plot that has in the Xlabel the datanum of all the month and in the Ylabel the distance in each time. My plot is the following way:

How can I write in the Xlabel just the days from 1-21 and to it more width so I can read easely the graph??(change the plot size(now is a square)).

[Image Analyst]

Please attach your x and y data and tell us what the x data represents.