Use a music editing software, or, choose a shorter song. You do know, I'm assuming, that in the project options you can click on "synchronize music to slides", don't you? The size of your files will end up being fairly large. What DPI are the images that are in your show, and, what are the physical size of them? Are they jpegs? Are they saved as a high quality jpeg, medium quality, or low quality? The music you are using, is it royalty free, or do you have a license to copy it?