If I make an animation with the stage size of 640x360 which is 16:9 and then when I export it to 1280x720 HD do I lose quality? It seems like it degrades a small amount. Would I get a higher quality outtput if I made the screen size 1280x720 to start? The artwork is vector. I export with the animation settings at 30 fps and the keyframe set at automatic using quicktime. I'm just trying to figure out the best setting for exporting Flash cartoon animation to video for use on YouTube or video.
You're always better off starting at your maximum desired target and scaling down from there. You do need to keep line thickness large enough to reduce safely however.
On the inverse, if you increase the size before export then expect that scale to evenly affect some objects in ways you may not desire, like stroke thickness. It depends where the artwork originates and what format it really is, like a smart object or a flash vector or imported EPS, etc etc. Text is always a challenge as well because it looks fine on a smaller size but can get undesirably large when upscaling (although it will render crisp).
Ultimately keeping strokes away from the 1px thickness and exporting to HD would be advisable. 1280x720 upscales to 1920x1080 fairly well, especially with vector, and text translates fairly well.