I am using Vdub: I am editing an avi, I went to filters and then resized the avi to crop out the black bars. Then I saved it, set to 'full processing mode'. But this file too big now, why can't I save it using 'direct stream copy' ?
You can, you just can't use filters. It isn't possible to crop, resize, etc. without recompressing (or in your case probably just decompressing) the video.
If you want to edit a video, you must use a codec (in Video___Compression) for compressing the Video. Do File___Information in VirtualDub. Let's suppose that AVI has 'Video compression: Xvid MPEG-2 Codec ; data rate: 740 kbps'. Compress that AVI using XviD multipass 740 kbps and the output file will be big as the source. You just have to have XviD installed. The same for DivX (with some little stuff to fix: you must tell it to use the Home Theater Profile + the 'slow' (maximum quality) method and, alas, it isn't free).