Fallout 4 window is too small. How do I change the size of my screen?

Before the game launches, where you select Options to change to Window Mode, you can adjust the resolution:

Otherwise, if you're trying to adjust the window to a more specific size, you can do so by editing Fallout4Prefs.ini. In Windows Explorer (I'm assuming Windows, if you use another OS I'm sure you can figure this out) navigate to your Fallout 4 saves folder, likely:

Documents\Fallout4

In here you'll find a file called Fallout4Prefs.ini. First make a backup in case you muck it up. Open this file in Notepad or your favourite text editor, and find the following values:

iSize H=450
iSize W=800

These refer to the height and width of your window, respectively. Change these values to whatever you would prefer. As an example:

iSize H=1280
iSize W=1900

Save the file, then relaunch Fallout 4. Once past the menu, you'll see the window is now at the size you set. Note these values only apply if you've selected Window Mode in Fallout 4