If you use the Save As option and choose JPEG from the options, you'll see a slider labelled Quality (IIRC). If you tick the preview box, you'll see the file size. If you zoom in on a critical part of the image before selecting Save As, the preview button will also show you what effect the compression is actually having on the final image. A first generation JPEG save from the original will show almost no degradation at a level of 10 and a print from a level of 8 will probably be indistinguishable from higher values.