I searched the site but did not find previous mention of this, so here it is.
Holding the Option key down while dragging to resize a QuickTime window will give you the (and I'm not sure of the technical term here) "non-interpolated sizes" available for that resolution, much in the same way the Dock behaves when you Option-drag its divider.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050512172427622