You can pass a special "auto" scale value to the me.video.init() function to automatically scale the canvas to any browser/screen size:
http://melonjs.github.io/docs/me.video.html#init This is fine for most cases.
I once tried making a game that could dynamically scale to the available screen size without stretching the canvas, but it turned out awful, because gameplay mechanics changed depending on the screen size. It was not good. Definitely recommend using the video scaling feature and live with lower quality graphics on higher resolution displays. (At least for your first game!) I'm going to attempt to do a game that can display low resolution and high resolution (2x pixel area) images depending on available screen space.