Believe it or not the dimensions of your FLV file can greatly improve (or reduce) playback quality and file size. The best case scenerio to to pic dimensions that are multiples of 16, for example 640×480.  Both numbers are divisible by 16.  If for some reason you can’t (or won’t) use multiples of 16, your next best choice is multiples of 8 or 4.

I’ve embeded a chart below that lists all suitable dimensions that are multiples of 16 (courtesy of Influxis Labs):

Another handy link is this Flash Video (FLV) Bitrate Calculator.  Great for determining an optimal bitrate.