There can be a noticable color shift from RGB to CMYK. One way of doing it is to stay in RGB, and avail youself of the filters etc, but use the CMYK colors. RGB or CMYK depends on the "printers" requirements. The image can/may "look" OK at lower res, but usually "Commerical" Printing requires 300 DPI. You can reduce the size of an image, (within reason) and not lose not lose shartness. Enlarging is another story. The greater the enlargement the more pixelated the image. The lower the res, the less you can enlarge. Say your doing an image at 300DPI or even 72DPI, If you enlarge some 72 DPI of portions of your image, and not others, some portions may look fuzzy/pixaliated and other sections may "look" OK, (on the monitor). If you plan to print the image larger than its current size, you need to increase the DPI above 300 DPI, (depending on the increase).