Because sometimes you'll wish somebody has made something to make your life a little bit easier, and you think, oh maybe I should actually make that myself then.
Layered PSD with many common aspect ratio. Download here
NOTE: these aren't the frame sizes, just the ratios - the PSD is based on a height of 1080px (the idea being that if you wanted to make a video for anything up to 2k you just need to fill the coloured box with video). I accept no responsibility for any inaccuracies!