At this time a viewable canvas size greater than 2048? would seriously harm performance.
2046, actually. iOS apparently adds two pixels to the edge of the texture to make it so that they can rotate without aliased edges.
Keep in mind when setting the resolution on a retina display device, you need to actually set it to a quarter (halve both the width and height) of what you want it set to, because iOS reports retina displays at a quarter their actual resolution. So to get 960x640, you set it to 480x320 (which would technically create a canvas of 482x322, but it doesn't matter since the extra pixels are blank/offscreen and the hardware can handle it, whereas the hardware cannot handle 2050x2050 textures).