Finding a 2D bounding box

How can I find the 2D bounding box of a 3D mesh as it is displayed on screen? It doesn't have to be really accurate but it does need to be fast. I don't want to have to loop through every vertex of each mesh every frame because it would be too slow.

Thanks.

Would MeshWidth/Height/Depth do it....??
EDIT: sorry, you want 2D box on screen

http://www.blitzbasic.com/Community/posts.php?topic=40417