In iDVD, go to Project > Project Info where the project can be set to Widescreen 16:9 or Standard 4:3.
However, I’ve found that if you create a 16:9 project and drop in a 4:3 QuickTime MOV (I forget the exact codec settings at the moment), iDVD correctly encodes it as 4:3 aspect with black sides, ie. not undesirably stretch it out to 16:9.