Click on that little command prompt looking icon in the upper left hand corner of the command prompt window. This should drop down a menu. Go to Properties then the Layout tab. Set the properties you desire for the window.
Hmmm, must be something else on your system that is resizing it as you launch it then.
You could alternatively click on Defaults instead of Properties in the drop down menu and see if that has a more lasting effect. I'm pretty sure that all that does is just take you to the Layout tab, however.
You could try clicking on Defaults, then go to the Options tab and select "Use legacy console (requires relaunch)".
After relaunch, click on Defaults, go to the Layout tab and adjust window size.