1280x720 is a bit low for today's monitors, I would choose at least 1920x1080. But if your game is played in a window or on a web page, 1280x720 may be enough.
The idea is that you choose one resolution and make all graphics assets for it. If the image in your sprite is 300x300 pixels and you have to resize it on the layout to make it 50x50 - you are wasting a lot of memory. If the image is too small and you are enlarging the sprite on the layout - you are losing graphics quality. So ideally most of your sprites should have 1:1 scale.