My monitor is 16:9, but all games I always make in 4:3, because the majority of casual games that I played has a standard 4:3 aspect ratio (640x480, 800x600, 1024x768). I think that casual games publishers are still making their games in 4:3.
For information: today i've had one experimental - recorded a 1 minute video in 720p 1280x720 (16:9), and the file size turned out 144 mb. <img src="http://i987.photobucket.com/albums/ae352/constructgame/fk.gif"> I think this is a very large size and we will have to make the video very short, but so many good games would like to show.
Presentation video by hero_bash is 7 min 27 sec, while the size is only 44 mb ( MPEG4 Video (H264) 656x480).
IMHO: I think our main task is to make a showreel, which will be available to most users that they all could appreciate Construct capabilities. If we make a video in HD 720p, then we significantly shorten the audience and only starred with high-speed Internet will able to watch it (even if people watch video in a low quality through YouTube option, it will reduce the size maximum of 2 times, ie will weigh about 60-70 mb, which is still a lot <img src="http://i987.photobucket.com/albums/ae352/constructgame/fw.gif"> ). Even if we are going to make in 16:9 - no more than WVGA 854x480 format, because the video size should not exceed 30-50 mb. Or better, I would suggest 800x600 (4:3).
I have a suggestion: let me capture a couple of my games and make two test video: the first in 4:3, the second in 16:9, and together we take a look and choose which format to use for the final video. Do you agree? <img src="http://i987.photobucket.com/albums/ae352/constructgame/gb.gif">