Adding Black Bars to make the correct size?

[humet]

Hi,

At the moment the AVIs I am converting to DVD always look a tiny bit stretched heightwise, even with the 16:9 flag and letterboxed flag checked.

My question is, is it possible to add tiny black bars to the top and bottom of the AVI before putting it through CCE, so that when the 16:9 and the letterbox flag are checked it looks the correct size on my TV.

If this is possible then how do I work out what size bars to put on the top and bottom?

Thanks

humet