I'm trying to render a composition that is 199x199. It is to be used as a flash movie on a website, so I rendered it as a .flv file. However, when I do this, the movie gets squeezed horizontally, leaving black bars on two sides.
As Rick said, check PAR. Naturally, since FLV is only meant for desktop playback, square pixels are all it will give you. Also change the size, if possible. Any block-based compression algorithm will have a hell of a time with such "crooked" dimensions (they should always be multiples of 2) and may introduce balck lines at the edges to fill in the missing pixels.