1920x1080 is a 16:9 screen ratio, so it should fit on most devices as well. However for tablest I personally recommend you to use a 16:10 screen ratio (1920x1200). Moreover it's not needed to built your mobile games in full hd, hd is sufficient. But this is up to you
Yes CocoonJS did support Letterbox scale, but CocoonIO doesn't.
Thank you for your feedback!
You recommedn 1920x1200 but in your blog post it says 1280x800, is that the same aspect ratio?
I recommend the user 1920x1200 because he has built his game in 1920x1080. But generally 1280x800 is sufficient.