In Movie Studio 8 I set my Project screen to 72x480 and it says it uder my little screen. But under that it say Preview Size and I for got how to change it?
Above the preview screen there is a button that says 'preview' with 4 options (draft upto best + auto/full/half/quarter). This affects the resolution of the preview screen.
Vegas is merely resizing the preview image to fit in your preview window, which must be 180x120 or so. This will not affect the quality of the output video.
If you want your preview to be higher resolution, make the preview window bigger.