What are the best Flash video frame dimensions?
Video codecs perform best when the frame width and height use multiples of 16. While you can use any width and height in your encoding settings, non-optimal dimensions can result in poor image quality and reduced frame rate. For the best image quality and playback, you should always use width and height dimensions that use a multiple of 16 (best), 8 (better), or 4 (good). We would recommend that you refrain from using 4 or 8 multiples unless it is absolutely necessary.
Note: Both the native encoding dimensions and the playback dimensions should be a multiple of 16. For example, if you encode your FLV file to use a frame dimension of 320 x 240, you should scale to a size such as 512 x 384.
Refer to the following tables to pick dimensions for your layout.
4:3 aspect ratio sizes:
16:9 aspect ratio sizes: