Encoding gurus have long known that to get optimal results from a codec it is best to choose a frame size that divisible by at least 4 or 8. This is because codecs often use use 4×4, 8×8 and sometimes 16×16 “blocks” to form an image. If your image dimensions are evenly divisible, no extra blocks are wasted and the codec can be most efficient with respect to image quality. In the case of On2 VP6 and Sorenson Spark codecs, both use 16×16 blocks. So to assist with choosing an optimal frame resolution, Adobe has shared a table of 4:3 and 16:9 encoding frame sizes that are useful for Flash video encoding. The table breaks down which frame resolutions are Good, Better, and Best (or, those that are divisible by 4, 8, or 16, respectively). They also have a handy Flash Bitrate Calculator that can help with other encoding and bitrate choices.

(Via askmrvideo)