Gadzooks! Found where the 320x240 size was coming from...

Apparently the constructor of the Video class has two optional parameters, width and height, that have default values of 320 and 240 respectively:
http://help.adobe.com/en_US/AS3LCR/F...dia/Video.html

So, when creating an instance of the Video class:

Actionscript Code:
var vid:Video = new Video(); // creates a 320x240 video
var vid:Video = new Video(960, 540); // creates a 960x540 video