"the output file from VirtualDubMod is a projected 250GB and I only have a 150GB drive ! Is there a way to modulate the size of the outputted file without compromising quality (too much ?). Probably a pipe dream, I know - but thought I'd ask"

I'm away from my main computer, but look under the Video menu (I think), and then Compression (someone please correct me.) You will see a list of codecs, and I'm betting that the first setting - Uncompressed/RGB - is selected. The codec of choice on these boards is called HuffyUV, and it'll get your filesize down well below 100GBs.