One of my favorite players; Crystal, comes in several flavors, there is a free no nagging version. Each version it loaded with features and one of the them I like is you can grab a corner of the window and drag it out to any size, even while the vid is playing and it reports the screen size back as you drag. No one player plays everything, this one seems to play some vids that others stumble on.

http://www.crystalplayer.com/index.php session=828437286c24c1e6436bea9d&page=cp