The question you are asking is a bit strange. The only answer I can think of is that your canvas/image size needs to be as big as you want it for whatever the end result is to be. If it is destined (only) for the web/monitor, then 72 DPI and an appropriate size (for monitors) is what you need to consider. If it is destined for print, that's a whole new "ballgame" that can lead to quite a discussion...but in general, it depends on the target size of the print. Welcome to the "land of Photoshop"!