The site will automatically display your game at the proper resolution, you don’t need to specify it. The only time it will change is if its wider than 700 pixels, at which point it will be scaled down.
EDIT: Looks like your movie is 870 pixels wide. Unfortunately thats too wide, and in the future you should likely design the game to be smaller. If your game is already running on high quality, set it to best instead (_quality=“best”) and make sure ‘allow smoothing’ is checked on your library bitmaps. That may possibly help.