I was just about to make a thread about this exact question. I tried searching but couldn't find an answer, but I decided to perform one last search, which was "custom extent dd", and I saw this thread.
*Sorry for resurrecting an old thread.
Thank you so much for posting the answer, this worked perfectly, I am providing my code below for anyone who is having difficulties. I too overlooked the AreaOfInterest property.
[PHP]
public static void SetDataFrameACustomFullExtent(IMapDocument pMapDocument)
{
IActiveView pActiveView = pMapDocument.ActiveView;
IEnvelope pEnvelope = new EnvelopeClass();
pEnvelope.XMax = 86.343146273; //Change these numbers to your numbers
pEnvelope.XMin = -86.343146273;
pEnvelope.YMax = 55.036972317;
pEnvelope.YMin = -55.036972317;
pMapDocument.Map[0].AreaOfInterest = pEnvelope;
}
[/PHP]
THANKS!