Whatever dpi you start with affects anything you do later. So if a map was made at 72 dpi and 10 inches by 10 inches, you can increase the resolution to 144 dpi, but that will cut the size to 5x5, or you can increase the size to 20 x 20 but that will cut the resolution to 36 dpi.

I usually make my maps at a higher resolution because I can always make them smaller or lower resolution without loosing anything if I need to.

