The best way to preview is to use this player:
VLC - (click on link) _ (choose nearest mirror site) as it has in-built codecs & is generally very reliable. Also, when I preview, I preview directly from the incomplete folder as that prevents a preview file being created.
Bitzi is a very good way of finding if there's been good or bad reports about a video;
How to use Bitzi Web Lookup
Other tips:
Movie download issues
Um ... iPod only has a small screen.
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But if you want the preview image to be larger in VLC, then Control-2, or 3 will do the trick.