Screen size doesn't matter as much when you use tablet, because you can map your active area to a specific screen region or to your entire screen.
For mouse, a lot of people recommend a smaller window size because it makes you have to move a smaller distance with the same sensitivity, but it also makes your targets smaller. When you play fullscreen or borderless you have more margin for error but you also have to move a greater distance.
It's really all about preference. I prefer playing fullscreen so I can focus more without being distracted by something outside of the window, like a new message notification popping up on Skype.