There are two parts to the answer.
The command shell option /k starts a new command prompt, runs some command (or a program, batch script, or group of commands) and the prompt remains rather than terminating.
In this case, the command is
mode, which can be used to set the command prompt screen size/window buffer so it's 200 columns wide and 70 rows high.
Finally, if you want to change the default size of the command prompt, right click on the title bar, select 'Properties', and select the 'Layout' tab. You can set the size of the screen buffer and the window separately. The width should be the same for both. The screen buffer height setting determines how far back you can scroll.