I have a Minolta Dimage

I have a Minolta Dimage S304 camera and I have a questio regarding image size a quality;
I am saving photos at an image size of 2046x1536, and the Fine Quality Setting.
When I load the pictures onto my computer and use photoshop to adjust them, the pictures are being saved at a size of 28 inches by 24 inches.
I then reduce the size to roughly 6 inches by 4 inches, prior to e-mailing them to Costco to print.
My problem is that when I reduce the image, and download it to Costco, I get a "Resolution warning" saying that the photos may appear blurry, grainy, or pixilated?
Any advice that you can provide to resolve this problem and allow my to get high quality prints of my photos will be greatly appreciated!!!!
Ralph Bischak
rbischak@comcast.net