Using DVDA Studio, click in File ---> Properties. Under Introduction Media, in the Video/Image section, hit the file folder icon and enter the video clip you want. Then do exactly the same for the audio. Enter the same file name and location there or your video clip will have no sound.
It's a bit more detailed in DVDA 2.0, but if you need the instructions, let me know.
There is no limit to how long the introduction (read: play without any menu) can be. I had over an hour of my brother-in-law playing golf in Florida as an introduction just so I could get around having a menu.