3MB doesn't mean a whole lot. What are the pixel dimensions? Generally the size of the picture should be no greater than what you need to have the smallest cropped (zoomed) section fill the output frame. If you're in NTSC-land then the frame size will be around 655x480 for 4:3 or 853x480 for widescreen. If you want to zoom in on a smaller area then make sure your image is large enough for that small area to be 480 pixels high.

Anything larger than this is pretty much a waste and can slow down Vegas a lot. Also, resizing them in a photo editor will probably give sharper and crisper results than having Vegas resize them when it renders. So, resizing them yourself is more likely to make the finished DVD look better than look worse.