Get the resolution of the projectior in question and match it.
If you don't know the resolution, to be modern, at least adhere to the commmon mini HD standard of 1280x720 (16:9). Many old companies may still have older 800x600 projectors but will play a scale reduced version of your stream. 1920x1080 proper projectors will play it doubled easily.
I've personally come across more companies with "InFocus" old projectors that I've set my standard at 1280x720 with a backup real 1920x1080 stream just incase they have full HD capacity.
sinious` recommendations are good, 1280x720 is the lowest common denominator currently.
The Flash fullscreen mode will scale your stage size down from that, even if you have to present on a 800x600 beamer, or up when you present on a 1680x1050.
1.If you want to make sure all your bitmaps look smooth in every scaling situation you have to enable bitmap.smooting in all the bitmaps of your library.
2.Use stage.scaleMode=StageScaleMode.SHOW_ALL to avoid distortion if the output ratio differs from your stage ratio.
3.Employ masks to avoid the situation that the viewer can see elements that you moved outside of the stage area.
24 out of 27 of the last companies in 2 years I've presented at were 4:3 or much more often 16:9. 1680x1050 is 8:5. I'd say that's least common denominator by far.
The sound advice is to force your presentation to run fullscreen while keeping aspect ratio (StageScaleMode.NO_BORDER). You should have a full HD (1920x1080) version at hand as well. When presenting, put your best foot forward and consider 1280x720 a worst case scenario.
If you want to follow sinious`advice to have a Full-HD (1080p)-video as best quality backup, make sure your presentation-rig has enough horsepower or have a look at Stage3D`s video option.
I´m currently making a presentation with a 1600x1200 stage and even on a modern quadcore I notice that the smoothness of Tweens (I´m using TweenMax) or the overuse of filters will drag down the software renderer to 20fps.
I have clients on dual core E8500s running 1920x1080, it's all in the quality of the code. One kiosk is on an AMD x2 4600 (8 years old?) with a radeon 9800 pro at HD. A quad core, especially modern as you say, is extreme overkill. Even though you don't need to have an intel quad (for turbo), AMD quad pipelines still fill efficiently. Dozens of product-specific kiosks I make for tradeshows all run on very minimal hardware with under 1GB free ram on very low end video cards (8800GT/4650HD/etc). The one thing you rarely get away with is using an Intel processor with integrated HD (2000/3000/4000) graphics as it runs fairly poorly.