It sounds as though you are on tablet mode rather than desktop mode, when you are in tablet mode, all apps take up the whole screen, it how they work best on most tablets especially if they are small.
Look along the taskbar at the bottom of your screen (Unless you have moved it somewhere else), over to the right, normally just by the system time and date is a small button with some squiggly lines, this is the action centre, if you click on that you should see a button for tablet mode, if you can't look for the word expand and see if it comes up, click on this and you will go to desktop mode and then the calculator should size normally.
You can go through the settings and set the PC to start in desktop or tablet mode later on.