DPI doesn't mean anything in this case, since we have no idea what an "inch" constitutes for your map or the page it sits on (and the word should be "unit", anyway). The total size of the file should be under the MB limits Rabulias has posted. Also, for a background map, don't use PNG. Use Jpg, since you don't need transparency and you can fiddle with the compression level to get a comfortable balance between size and quality. Finally, make sure your image is labeled .jpg, not .jp e g, since the latter causes issues on Roll20.
